Timeline: return 'null' from JSCallStack::asTraceFormat() when stack is absent

Timeline: return 'null' from JSCallStack::asTraceFormat() when stack is absent

Timeline front-end used to assume that if call stack is present, it contains at
least one call frame. Let's not break that expectation when transitioning to
trace-based instrumentation and return null, not empty array, for the absent stack.

BUG=361045
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175898

[yurys, 2014-06-10 11:58:47]

Is there a scenario where we can see the difference in behavior because of this
change?

[caseq, 2014-06-10 12:01:48]

On 2014/06/10 11:58:47, yurys wrote:
> Is there a scenario where we can see the difference in behavior because of
this
> change?

Yes. Navigate to http://lenta.ru, start timeline, scroll once, stop timeline.
Before this change, we would crash since linkifyTopCallFrame() assumes that if
the call stack is present, it has at least one frame.

On a second thought, let's also fix this in the front-end.

[yurys, 2014-06-10 12:33:49]

lgtm

[caseq, 2014-06-10 12:38:17]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-06-10 12:38:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/caseq@chromium.org/327793002/20001

[commit-bot: I haz the power, 2014-06-10 14:31:25]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/11430)

[commit-bot: I haz the power, 2014-06-10 15:32:33]

Change committed as 175898