Got it here:
https://build.chromium.org/p/tryserver.chromium.angle/builders/mac_angle_rel_ng/builds/2993
https://chromium-swarm.appspot.com/task?id=33371dbb8b29fa10&refresh=10&show_raw=1

WebglConformance_deqp_functional_gles3_shaderstruct (gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest) ... Received signal 11 SEGV_MAPERR 125a89d00000
[0x000102673476]
[0x7fff81db2f1a]
[0x7f84db417ff0]
[0x000100ca9495]
[0x000100ca0b8d]
[0x000100ca0f9d]
[0x000100cb39bd]
[0x000102730761]
[0x000102721daf]
[0x7fff8442d05a]
[0x7fff8442cfd7]
[0x7fff8442a3ed]
[end of stack trace]
(INFO) 2016-12-20 14:16:59,776 desktop_browser_backend._GetAllCrashpadMinidumps:368 Found crashpad_database_util
(INFO) 2016-12-20 14:16:59,791 desktop_browser_backend.GetStackTrace:542 Minidump found: /b/s/w/itNDQke3/tmpnx0F51/completed/838392f6-ce10-4670-86c0-fc2c42c8f916.dmp
(INFO) 2016-12-20 14:16:59,791 cloud_storage.Insert:312 Uploading /b/s/w/itNDQke3/tmpnx0F51/completed/838392f6-ce10-4670-86c0-fc2c42c8f916.dmp to gs://chrome-telemetry-output/minidump-2016-12-20_14-16-59-238056.dmp
(INFO) 2016-12-20 14:17:01,881 cloud_storage._GetLocked:274 Downloading gs://chromium-telemetry/binary_dependencies/minidump_stackwalk_76c5983fc9e9316a9d4251ba3e68b955c4fc9bf3 to /b/s/w/irxcDMqe/third_party/catapult/telemetry/telemetry/internal/bin/mac/x86_64/minidump_stackwalk
(INFO) 2016-12-20 14:17:02,425 desktop_browser_backend.GenerateBreakpadSymbols:79 Dumping breakpad symbols.
(INFO) 2016-12-20 14:17:02,426 cloud_storage._GetLocked:274 Downloading gs://chromium-telemetry/binary_dependencies/minidump_dump_c39bd7a3b9fa6279893b2d759045699d79ce4dcb to /b/s/w/irxcDMqe/third_party/catapult/telemetry/telemetry/internal/bin/mac/x86_64/minidump_dump
(INFO) 2016-12-20 14:17:16,802 desktop_browser_backend._GetAllCrashpadMinidumps:368 Found crashpad_database_util
Can't get standard output with --show-stdout

Traceback (most recent call last):
_RunGpuTest at content/test/gpu/gpu_tests/gpu_integration_test.py:78
self.RunActualGpuTest(url, *args)
RunActualGpuTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:196
getattr(self, test_name)(test_path, *args[1:])
_RunConformanceTest at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:210
self._CheckTestCompletion()
_CheckTestCompletion at content/test/gpu/gpu_tests/webgl_conformance_integration_test.py:204
'webglTestHarness._finished', timeout_in_seconds=300)
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
WaitForJavaScriptCondition at third_party/catapult/telemetry/telemetry/internal/actions/action_runner.py:246
self._tab.WaitForJavaScriptExpression(condition, timeout_in_seconds)
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
WaitForJavaScriptExpression at third_party/catapult/telemetry/telemetry/internal/browser/web_contents.py:130
py_utils.WaitFor(IsJavaScriptExpressionTrue, timeout)
WaitFor at third_party/catapult/common/py_utils/py_utils/__init__.py:120
res = condition()
IsJavaScriptExpressionTrue at third_party/catapult/telemetry/telemetry/internal/browser/web_contents.py:123
return bool(self.EvaluateJavaScript(expr))
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
EvaluateJavaScript at third_party/catapult/telemetry/telemetry/internal/browser/web_contents.py:194
expr, context_id=None, timeout=timeout)
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
EvaluateJavaScriptInContext at third_party/catapult/telemetry/telemetry/internal/browser/web_contents.py:222
expr, context_id=context_id, timeout=timeout)
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
inner at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py:36
return func(inspector_backend, *args, **kwargs)
EvaluateJavaScript at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py:211
return self._runtime.Evaluate(expr, context_id, timeout)
Evaluate at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_runtime.py:45
res = self._inspector_websocket.SyncRequest(request, timeout)
SyncRequest at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_websocket.py:110
res = self._Receive(timeout)
_Receive at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_websocket.py:166
self._HandleNotification(result)
_HandleNotification at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_websocket.py:179
self._domain_handlers[domain_name](result)
traced_function at third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py:52
return func(*args, **kwargs)
_HandleInspectorDomainNotification at third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py:367
raise exception
DevtoolsTargetCrashException: Devtools target crashed
********************************************************************************
(/b/s/w/irxcDMqe/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py:420 _AddDebuggingInformation) Received a socket error in the browser connection and the tab no longer exists. The tab probably crashed.
********************************************************************************
(/b/s/w/irxcDMqe/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py:421 _AddDebuggingInformation) Debugger url: ws://127.0.0.1:55490/devtools/page/4b3725c9-c3b1-4cc5-978b-777d982b2446
Found Minidump: True
Stack Trace:
********************************************************************************
Operating system: Mac OS X
10.10.5 14F1808
CPU: amd64
family 6 model 69 stepping 1
4 CPUs

GPU: UNKNOWN

Crash reason: EXC_BAD_ACCESS / KERN_INVALID_ADDRESS
Crash address: 0x125a89d00000
Process uptime: 163 seconds

Thread 16 (crashed)
0 Chromium Framework!__ZN2v88internal18LiveObjectIteratorILNS0_23LiveObjectIterationModeE0EE4NextEv + 0xf8
rax = 0x00000000beadbeee rdx = 0x0000000000000002
rcx = 0x0000000000000000 rbx = 0x0000125a89d00000
rsi = 0x00001bdb6c4823b9 rdi = 0x0000125a89d00001
rbp = 0x0000000120c38bd0 rsp = 0x0000000120c38ba0
r8 = 0x0000000000000039 r9 = 0x000000000000001f
r10 = 0x000000000000003f r11 = 0x0000000000000002
r12 = 0x0000125a89cfff60 r13 = 0x0000000120c38be0
r14 = 0x00001bdb6c4823b9 r15 = 0x0000125a89cffff8
rip = 0x0000000100ca9a48
Found by: given as instruction pointer in context
1 Chromium Framework!__ZN2v88internal20MarkCompactCollector7Sweeper8RawSweepEPNS0_4PageENS2_22FreeListRebuildingModeENS2_22FreeSpaceTreatmentModeE + 0x445
rbp = 0x0000000120c38cb0 rsp = 0x0000000120c38be0
rip = 0x0000000100ca9495
Found by: previous frame's frame pointer
2 Chromium Framework!__ZN2v88internal20MarkCompactCollector7Sweeper17ParallelSweepPageEPNS0_4PageENS0_15AllocationSpaceE + 0x6d
rbp = 0x0000000120c38cf0 rsp = 0x0000000120c38cc0
rip = 0x0000000100ca0b8d
Found by: previous frame's frame pointer
3 Chromium Framework!__ZN2v88internal20MarkCompactCollector7Sweeper18ParallelSweepSpaceENS0_15AllocationSpaceEii + 0x1ad
rbp = 0x0000000120c38d50 rsp = 0x0000000120c38d00
rip = 0x0000000100ca0f9d
Found by: previous frame's frame pointer
4 Chromium Framework!__ZN2v88internal20MarkCompactCollector7Sweeper11SweeperTask3RunEv + 0x4d
rbp = 0x0000000120c38d80 rsp = 0x0000000120c38d60
rip = 0x0000000100cb39bd
Found by: previous frame's frame pointer
5 Chromium Framework!__ZN4base12_GLOBAL__N_112WorkerThread10ThreadMainEv + 0x281
rbp = 0x0000000120c38ec0 rsp = 0x0000000120c38d90
rip = 0x0000000102730761
Found by: previous frame's frame pointer
6 Chromium Framework!__ZN4base12_GLOBAL__N_110ThreadFuncEPv + 0x5f
rbp = 0x0000000120c38ef0 rsp = 0x0000000120c38ed0
rip = 0x0000000102721daf
Found by: previous frame's frame pointer
7 libsystem_pthread.dylib + 0x405a
rbp = 0x0000000120c38f10 rsp = 0x0000000120c38f00
rip = 0x00007fff8442d05a
Found by: previous frame's frame pointer
8 libsystem_pthread.dylib + 0x3fd7
rbp = 0x0000000120c38f50 rsp = 0x0000000120c38f20
rip = 0x00007fff8442cfd7
Found by: previous frame's frame pointer
9 libsystem_pthread.dylib + 0x13ed
rbp = 0x0000000120c38f78 rsp = 0x0000000120c38f60
rip = 0x00007fff8442a3ed
Found by: previous frame's frame pointer
10 Chromium Framework!__ZN4base14PlatformThread6DetachENS_20PlatformThreadHandleE + 0x70
rsp = 0x0000000120c39030 rip = 0x0000000102721d50
Found by: stack scanning

hpayer@, seems like you've changed this code recently in  issue 630386 , https://codereview.chromium.org/2210493002, could you please take a look?