Add trace event for detecting main frame in loading metrics.

third_party/WebKit/public/web/WebLocalFrame.h

Index: third_party/WebKit/public/web/WebLocalFrame.h
diff --git a/third_party/WebKit/public/web/WebLocalFrame.h b/third_party/WebKit/public/web/WebLocalFrame.h
index 7f51aa1d49449953e6a18a00f4001a19ff8fb31b..b4ee7153ebf1baf4c45ec867399fbf2ff896b6f7 100644
--- a/third_party/WebKit/public/web/WebLocalFrame.h
+++ b/third_party/WebKit/public/web/WebLocalFrame.h
@@ -502,6 +502,12 @@ class WebLocalFrame : public WebFrame {
   // Returns the WebInputMethodController associated with this local frame.
   virtual WebInputMethodController* inputMethodController() const = 0;
 
+  // Tracing support ----------------------------------------------------------
+  // Returns the value for the "frame" argument in TRACE_EVENT* calls.
+  // This function is intended to be used in components that do not have
+  // access to the local frame, but still want to trace frame related events.
+  virtual void* frameIdForTracing() const = 0;

[ulan, 2017/02/22 21:16:37]

This is not really nice. I would appreciate better suggestions.

The problem is that the existing TRACE_EVENTs use the address of the local frame
as the id (BTW, does it work if Oilpan does compaction?). 

The RenderFrameImpl doesn't seem to have access to the local frame, so we need
to either pull the frame id from Blink or push a bit to Blink saying whether the
frame is the main frame or not.

[haraken, 2017/02/22 23:56:21]

In practice, this will work because Oilpan's compaction moves only backing
stores of vectors, hashtables etc. Oilpan doesn't move normal objects like
Frame.

However, it's not nice to expose the frame ID via the public APIs.

+dcheng@ might have some ideas.

+
  protected:
   explicit WebLocalFrame(WebTreeScopeType scope) : WebFrame(scope) {}