[api] Do not force external GCs when only trying to synchronously process phantom callbacks

[api] Do not force external GCs when only trying to synchronously process phantom callbacks

Improves on aec8987b5ee55fa46b90b58927e957a4e5f19c09 by not forcing external GCs
(blink) through the GC epilogue callback.

BUG=chromium:515795
LOG=N

Committed: https://crrev.com/3aca47b7aa4bf2d9156d62b9d6bf27a6761890c9
Cr-Commit-Position: refs/heads/master@{#30164}

[Michael Lippautz, 2015-08-13 15:23:33]

mlippautz@chromium.org changed reviewers:
+ jochen@chromium.org

[Michael Lippautz, 2015-08-13 15:23:33]

Jochen: The change to force a synchronously processing phantom callbacks had the
side effect of forcing GCs in blink, adding up to the atomic pause in V8's full
GC. 

smoothness.image_decoding_cases still ok on N9 with this change.

Hannes: Rips off ~6-9ms accounted to external from the full GC pauses in oort.

https://codereview.chromium.org/1287323002/diff/1/include/v8.h
File include/v8.h (left):

https://codereview.chromium.org/1287323002/diff/1/include/v8.h#oldcode4968
include/v8.h:4968: kGCCallbackFlagCompacted = 1 << 0,
Can we remove this unused flag?

[jochen (gone - plz use gerrit), 2015-08-13 15:26:14]

https://codereview.chromium.org/1287323002/diff/1/src/global-handles.cc
File src/global-handles.cc (right):

https://codereview.chromium.org/1287323002/diff/1/src/global-handles.cc#newco...
src/global-handles.cc:887: kGCCallbackFlagSynchronousPhantomCallbackProcessing)
!= 0;
if FlagForced is set, we also need to do the second callback synchronously

[Michael Lippautz, 2015-08-13 16:33:16]

PTAL

https://codereview.chromium.org/1287323002/diff/1/src/global-handles.cc
File src/global-handles.cc (right):

https://codereview.chromium.org/1287323002/diff/1/src/global-handles.cc#newco...
src/global-handles.cc:887: kGCCallbackFlagSynchronousPhantomCallbackProcessing)
!= 0;
On 2015/08/13 15:26:14, jochen (ooo) wrote:
> if FlagForced is set, we also need to do the second callback synchronously

Done.

[jochen (gone - plz use gerrit), 2015-08-13 16:36:35]

lgtm

[Michael Lippautz, 2015-08-13 16:41:27]

The CQ bit was checked by mlippautz@chromium.org

[commit-bot: I haz the power, 2015-08-13 16:41:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287323002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287323002/20001

[commit-bot: I haz the power, 2015-08-13 17:22:53]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-13 17:23:10]

Patchset 2 (id:??) landed as
https://crrev.com/3aca47b7aa4bf2d9156d62b9d6bf27a6761890c9
Cr-Commit-Position: refs/heads/master@{#30164}