[wrapper-tracing] Avoid GC during PostMessageTimer construction

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

Index: third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
diff --git a/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp b/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
index aad31716786f96c5a49adbea8bb9c4b5932a7395..86f2a58cdd13d15cae46cec45c18279a660a36d9 100644
--- a/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
@@ -625,9 +625,10 @@ void LocalDOMWindow::schedulePostMessage(MessageEvent* event,
   // Allowing unbounded amounts of messages to build up for a suspended context
   // is problematic; consider imposing a limit or other restriction if this
   // surfaces often as a problem (see crbug.com/587012).
-  PostMessageTimer* timer = new PostMessageTimer(
-      *this, event, std::move(target), SourceLocation::capture(source),
-      UserGestureIndicator::currentToken());
+  std::unique_ptr<SourceLocation> location = SourceLocation::capture(source);

[sof, 2016/11/29 09:15:44]

Subtle staging of allocations going on here wrt 'new'; an explanatory comment
covering why the allocation needs to be lifted out, would have been helpful.

Even better, following the standard pattern of having a create() factory method
would have prevented the problem from arising, i think.

[Michael Lippautz, 2016/11/29 09:51:41]

Sorry, for not leaving a comment. The rationale is that we cannot call into V8
from a gcForbidden scope, because V8 can now trace through blink, effectively
requiring some of its marking infrastructure.
I will add (D)CHECKs that would immediately fail (even without triggering a GC)
when calling into V8 from such scopes. Only those call sites require
refactoring. 

Using ::create might mitigate this scenario, but people could still call it from
ctors before vtable poitners are available.

+  PostMessageTimer* timer =
+      new PostMessageTimer(*this, event, std::move(target), std::move(location),
+                           UserGestureIndicator::currentToken());
   timer->startOneShot(0, BLINK_FROM_HERE);
   timer->suspendIfNeeded();
   m_postMessageTimers.add(timer);