devtools crashes when tring to generate stack trace in some cases

[rogerwang]

It tries to convert externalized string:

#0 0x08702a04 in WTF::StringImpl::ref (this=0xfa2)
at ../../third_party/WebKit/Source/WTF/wtf/text/StringImpl.h:581
#1 0x08702cbc in WTF::refIfNotNullWTF::StringImpl (ptr=0xfa2)

at ../../third_party/WebKit/Source/WTF/wtf/PassRefPtr.h:47

#2 0x0930bd77 in WTF::RefPtrWTF::StringImpl::RefPtr (this=0xffff25f4, o=...)

at ../../third_party/WebKit/Source/WTF/wtf/RefPtr.h:43

#3 0x0930b06a in WTF::String::String (this=0xffff25f4)

at ../../third_party/WebKit/Source/WTF/wtf/text/WTFString.h:103

#4 0x097e756a in WebCore::v8StringToWebCoreStringWTF::String (v8String=...,

external=WebCore::Externalize)
at ../../third_party/WebKit/Source/WebCore/bindings/v8/V8StringResource.cpp:119

#5 0x0979d72e in WebCore::V8StringResource<(WebCore::V8StringResourceMode)0>::toStringWTF::String (this=0xffff2594)

at ../../third_party/WebKit/Source/WebCore/bindings/v8/V8StringResource.h:240

#6 0x0979d408 in WebCore::V8StringResource<(WebCore::V8StringResourceMode)0>::operator WTF::String (this=0xffff2594)

at ../../third_party/WebKit/Source/WebCore/bindings/v8/V8StringResource.h:202

#7 0x0979c77e in WebCore::toWebCoreString (value=...)

at ../../third_party/WebKit/Source/WebCore/bindings/v8/V8Binding.h:105

#8 0x09bfcddc in WebCore::toScriptCallFrame (frame=...)

at ../../third_party/WebKit/Source/WebCore/bindings/v8/ScriptCallStackFactory.cpp:55

#9 0x09bfcf8c in WebCore::toScriptCallFramesVector (stackTrace=...,

scriptCallFrames=WTF::Vector of length 7, capacity 16 = {...}, 
maxStackSize=200, emptyStackIsAllowed=true)
at ../../third_party/WebKit/Source/WebCore/bindings/v8/ScriptCallStackFactory.cpp:75

#10 0x09bfd100 in WebCore::createScriptCallStack (stackTrace=...,

maxStackSize=200, emptyStackIsAllowed=true)
at ../../third_party/WebKit/Source/WebCore/bindings/v8/ScriptCallStackFactory.cpp:90

#11 0x09bfd1ed in WebCore::createScriptCallStack (maxStackSize=200,

emptyStackIsAllowed=true)
at ../../third_party/WebKit/Source/WebCore/bindings/v8/ScriptCallStackFactory.cpp:105

#12 0x099bfe2e in WebCore::InspectorResourceAgent::buildInitiatorObject (this=

---Type to continue, or q to quit---
0xf63d900, document=0xf670000)
at ../../third_party/WebKit/Source/WebCore/inspector/InspectorResourceAgent.cpp:455
#13 0x099be9d2 in WebCore::InspectorResourceAgent::willSendRequest (this=

0xf63d900, identifier=29, loader=0xf6ae800, request=..., 
redirectResponse=...)
at ../../third_party/WebKit/Source/WebCore/inspector/InspectorResourceAgent.cpp:235

#14 0x09986c49 in WebCore::InspectorInstrumentation::willSendRequestImpl (

instrumentingAgents=0xf63dae0, identifier=29, loader=0xf6ae800, 
request=..., redirectResponse=...)
at ../../third_party/WebKit/Source/WebCore/inspector/InspectorInstrumentation.cpp:664

[rogerwang]

fixed with the previous commit.