First build in which this was noticed:

https://build.chromium.org/p/chromium.gpu/builders/Win7%20Debug%20%28NVIDIA%29/builds/39477

Error:

[ RUN      ] Maps.maps_002
V8 error: Could not convert to object (v8::Object::Cast()).  Current memory usage: 429 MB
Backtrace:
	blink::MainThreadDebugger::isWorker [0x23A6FD19+5065]
	v8::FunctionTemplate::RemovePrototype [0x0F4B355F+255]
	blink::FetchDataLoader::Client::operator= [0x1EBDB48D+1334215]
	blink::FetchDataLoader::Client::operator= [0x1F1B3C01+7463739]
	blink::ScriptValueSerializer::doSerialize [0x23A3404C+300]
	blink::ScriptValueSerializer::serialize [0x23A3C4EB+91]
	blink::FetchDataLoader::Client::operator= [0x1F1BA2AA+7490020]
	blink::SerializedScriptValueFactory::doSerialize [0x23A4C923+67]
	blink::FetchDataLoader::Client::operator= [0x1F1B9B75+7488175]
	blink::SerializedScriptValueFactory::create [0x23A4BE84+52]
	blink::PointerEventInit::pointerType [0x23B6DA8A+538]
	blink::PointerEventInit::pointerType [0x23B6DBC9+857]
	std::vector<v8::CpuProfileDeoptFrame,std::allocator<v8::CpuProfileDeoptFrame> >::push_back [0x0F7AA4F8+792]
	std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy [0x0F5C4CE9+17417]
	std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy [0x0F5C4CFE+17438]
	v8::ResourceConstraints::max_semi_space_size [0x0F9839C7+435895]
	std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy [0x0F5CF6D9+60921]

...

Found Minidump: True
Stack Trace:
********************************************************************************
	ChildEBP RetAddr  
	WARNING: Stack unwind information not available. Following frames may be wrong.
	0018d958 0f44355f webcore_shared!blink::MainThreadDebugger::isWorker+0x13c9
	*** WARNING: Unable to verify checksum for modules.dll
	*** ERROR: Symbol file could not be found.  Defaulted to export symbols for modules.dll - 
	0018d98c 1ebdb48d v8!v8::FunctionTemplate::RemovePrototype+0xff
	0018d9a0 1f1b3c01 modules!blink::FetchDataLoader::Client::operator=+0x145bc7
	0018d9f0 23a3404c modules!blink::FetchDataLoader::Client::operator=+0x71e33b
	0018da18 23a3c4eb webcore_shared!blink::ScriptValueSerializer::doSerialize+0x12c
	0018da4c 1f1ba2aa webcore_shared!blink::ScriptValueSerializer::serialize+0x5b
	0018db08 23a4c923 modules!blink::FetchDataLoader::Client::operator=+0x7249e4
	0018db38 1f1b9b75 webcore_shared!blink::SerializedScriptValueFactory::doSerialize+0x43
	0018dbd4 23a4be84 modules!blink::FetchDataLoader::Client::operator=+0x7242af
	0018dbfc 239baa61 webcore_shared!blink::SerializedScriptValueFactory::create+0x34
	0018dcc4 23c4395c webcore_shared!blink::ChromeClient::postAccessibilityNotification+0x2f1
	0018dcd0 0f73a4f8 webcore_shared!blink::SecurityPolicyViolationEventInit::originalPolicy+0x6cc
	0018dd10 0f554ce9 v8!std::vector<v8::CpuProfileDeoptFrame,std::allocator<v8::CpuProfileDeoptFrame> >::push_back+0x318
	0018dd24 0f554cfe v8!std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy+0x4409
	0018ddac 0f55f6d9 v8!std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy+0x441e
	00000000 00000000 v8!std::vector<v8::CpuProfileDeoptInfo,std::allocator<v8::CpuProfileDeoptInfo> >::_Buy+0xedf9
********************************************************************************


Unfortunately the stack trace isn't symbolized well (see  Issue 605637 , just uploaded) but I suspect this CL:

https://codereview.chromium.org/1862033002