Make WindowReachedScreen support async readback

Make WindowReachedScreen support async readback

This is the final caller that doesn't fall back or is Aura-only, and so
Aura's snapshot can switch over to only being asynchronous.

BUG=332167
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245354

[enne (OOO), 2014-01-08 01:44:33]

PTAL if you get a chance.  There are some dependent patches so this isn't urgent
(and might change slightly), but I wanted to get a sanity check since I don't
know this code very well.

I tested this on a Linux ChromeOS build and verified that the async path was
being taken (and the test still passed).  I also tested this on a Linux non-Aura
GTK build and verified that the sync path was still being taken (and the test
still passed).

Depends on https://codereview.chromium.org/119753007/ (for the async callback),
https://codereview.chromium.org/126373002/, and
https://codereview.chromium.org/126373002/ (to remove the penultimate caller in
Aura of the sync window snapshot function).

[Ken Russell (switch to Gerrit), 2014-01-08 03:38:05]

This LGTM if it passes Telemetry's unit tests. Please send it through the GPU
try bots in addition to the normal set. Testing a Linux Aura build in addition
to the Chrome OS build would be a good idea.

[bajones, 2014-01-08 17:41:21]

On 2014/01/08 03:38:05, Ken Russell wrote:
> This LGTM if it passes Telemetry's unit tests. Please send it through the GPU
> try bots in addition to the normal set. Testing a Linux Aura build in addition
> to the Chrome OS build would be a good idea.

LGTM too, though I'm not an owner of snapshot/ sorry.

[enne (OOO), 2014-01-09 02:07:50]

Thanks for the suggestions.  Telemetry unit tests pass as much as they did
before.  Two fail (BrowserTest.testMultipleTabCalls and
TemporaryHTTPServerTest.testBasicHostingAndRangeRequests), but also fail without
this patch.  Linux non-ChromeOS Aura build still passes run_gpu_test pixel as
well.

ben: OWNERS for ui/snapshot and content/browser/
sergeyu: FYI for more async snapshot changes

[Ben Goodger (Google), 2014-01-09 23:11:55]

lgtm

[commit-bot: I haz the power, 2014-01-16 20:42:23]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/enne@chromium.org/127103002/490001

[commit-bot: I haz the power, 2014-01-16 22:36:20]

Change committed as 245354