Fix <webview> context menu positioning on mac.

Fix <webview> context menu positioning on mac.

Something changed along the way and it seems the coordinate translation
is necessary for mac too.

BUG=406117
Test=Open a <webview> in a chrome app.
Right click on the <webview>, context menu should appear in correct
position, regardless of the position of the <webview> relative to the
embedder.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291438

[lazyboy, 2014-08-21 23:51:28]

FYI, I didn't do any investigation why this is happening since I found the quick
fix to work.

[sky, 2014-08-22 00:07:22]

+avi in case he knows.

[Avi (use Gerrit), 2014-08-22 00:20:10]

On 2014/08/22 00:07:22, sky wrote:
> +avi in case he knows.

No clue. If this fixes things, then sure, LGTM, though it's weird that it's just
happening now.

[sky, 2014-08-22 15:54:49]

LGTM

[lazyboy, 2014-08-22 16:15:55]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-08-22 16:16:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/lazyboy@chromium.org/490293003/1

[commit-bot: I haz the power, 2014-08-22 17:12:50]

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

[commit-bot: I haz the power, 2014-08-22 17:20:17]

Committed patchset #1 (1) as 291438