sync compositor: Make OnComputeScroll async IPC

sync compositor: Make OnComputeScroll async IPC

OnComputeScroll can synchronously update the scroll offset. However, it
is called immediately before draw, which is synchronous and synchronizes
the scroll offset. So UI scroll offset is stale for only a short time.

Note that stale scroll offset from UI is ignored during draw, so this
does not break flings.

BUG=545633
Committed: https://crrev.com/7f873dd7d474ed8e7ba9287d2e61f4ad180f0884
Cr-Commit-Position: refs/heads/master@{#370808}

[boliu, 2016-01-20 19:33:38]

boliu@chromium.org changed reviewers:
+ dcheng@chromium.org, hush@chromium.org

[boliu, 2016-01-20 19:33:38]

ptal
dcheng for ipc
hush for everything else

[hush (inactive), 2016-01-21 00:18:27]

On 2016/01/20 19:33:38, boliu wrote:
> ptal
> dcheng for ipc
> hush for everything else

A question I had in mind:
Since computeScroll is now an async IPC, is it possible that the renderer
process receives the computeScroll IPC *after* the sync IPC for the onDraw? If
that happens, that might break things

[boliu, 2016-01-21 00:19:26]

On 2016/01/21 00:18:27, hush wrote:
> On 2016/01/20 19:33:38, boliu wrote:
> > ptal
> > dcheng for ipc
> > hush for everything else
> 
> A question I had in mind:
> Since computeScroll is now an async IPC, is it possible that the renderer
> process receives the computeScroll IPC *after* the sync IPC for the onDraw? If
> that happens, that might break things

Can't happen. Renderer side receive all IPCs in order, sync or async. :D

[hush (inactive), 2016-01-21 00:21:09]

On 2016/01/21 00:19:26, boliu wrote:
> On 2016/01/21 00:18:27, hush wrote:
> > On 2016/01/20 19:33:38, boliu wrote:
> > > ptal
> > > dcheng for ipc
> > > hush for everything else
> > 
> > A question I had in mind:
> > Since computeScroll is now an async IPC, is it possible that the renderer
> > process receives the computeScroll IPC *after* the sync IPC for the onDraw?
If
> > that happens, that might break things
> 
> Can't happen. Renderer side receive all IPCs in order, sync or async. :D

That's great.
lgtm. Thanks!

[dcheng, 2016-01-21 21:23:09]

lgtm

[boliu, 2016-01-21 21:24:08]

The CQ bit was checked by boliu@chromium.org

[commit-bot: I haz the power, 2016-01-21 21:26:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1601743008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1601743008/1

[commit-bot: I haz the power, 2016-01-21 22:37:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-01-21 22:38:36]

Description was changed from

==========
sync compositor: Make OnComputeScroll async IPC

OnComputeScroll can synchronously update the scroll offset. However, it
is called immediately before draw, which is synchronous and synchronizes
the scroll offset. So UI scroll offset is stale for only a short time.

Note that stale scroll offset from UI is ignored during draw, so this
does not break flings.

BUG=545633
==========

to

==========
sync compositor: Make OnComputeScroll async IPC

OnComputeScroll can synchronously update the scroll offset. However, it
is called immediately before draw, which is synchronous and synchronizes
the scroll offset. So UI scroll offset is stale for only a short time.

Note that stale scroll offset from UI is ignored during draw, so this
does not break flings.

BUG=545633

Committed: https://crrev.com/7f873dd7d474ed8e7ba9287d2e61f4ad180f0884
Cr-Commit-Position: refs/heads/master@{#370808}
==========

[commit-bot: I haz the power, 2016-01-21 22:38:36]

Patchset 1 (id:??) landed as
https://crrev.com/7f873dd7d474ed8e7ba9287d2e61f4ad180f0884
Cr-Commit-Position: refs/heads/master@{#370808}