http/.../mutation-observer-suspend-while-paused.js in webkit_layout_tests failing on multiple builders

Builders failed on: 
- WebKit Mac10.11: 
  https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.11
- WebKit Mac10.12: 
  https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.12


The text diff of the test is different than expected:
(Copied from: https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=79306c82d3b607790bd2c0c80b36661eb78bfc78&as=mutation-observer-suspend-while-paused-diff.txt)

--- /b/s/w/ioYpnmDj/layout-test-results/http/tests/devtools/sources/debugger/mutation-observer-suspend-while-paused-expected.txt
+++ /b/s/w/ioYpnmDj/layout-test-results/http/tests/devtools/sources/debugger/mutation-observer-suspend-while-paused-actual.txt
@@ -1,3 +1,5 @@
+CONSOLE ERROR: line 57: Uncaught TypeError: Cannot read property 'dispatchMessage' of undefined
+CONSOLE ERROR: line 57: Uncaught TypeError: Cannot read property 'dispatchMessage' of undefined
 Tests that DOM Mutation Observers do not attempt to deliver mutation records while the debugger is paused. Bug 105810 
 
 DIV and observer setup.


Will: Would you please help to triage this bug? I haven't seen anything obvious in the blamelist.
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.12/builds/7666