dragend fix for mac <webview>.

dragend fix for mac <webview>.

dragend would never fire on mac because
BrowserPluginEmbedder::SystemDragEnded() clears |guest_dragging_over_|, which stops
BrowserPluginEmbedder::DragSourceEndedAt() to send the dragend
message.
This isn't an issue in aura b/c, we see DrageSourceEndedAt() before
SystemDragEnded().

BUG=401941
Test=On mac, load a <webview> in a chrome app, register a "dragend"
listener to a draggable div. Drag that div, observe "dragend" listener
firing properly.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291356

[Fady Samuel, 2014-08-21 23:36:31]

https://chromiumcodereview.appspot.com/499493002/diff/20001/content/browser/b...
File content/browser/browser_plugin/browser_plugin_embedder.cc (right):

https://chromiumcodereview.appspot.com/499493002/diff/20001/content/browser/b...
content/browser/browser_plugin/browser_plugin_embedder.cc:32:
seen_drag_source_ended_at_(false),
Can we do this with one flag?

[lazyboy, 2014-08-22 00:03:27]

Comments addressed in patchset #3

https://chromiumcodereview.appspot.com/499493002/diff/20001/content/browser/b...
File content/browser/browser_plugin/browser_plugin_embedder.cc (right):

https://chromiumcodereview.appspot.com/499493002/diff/20001/content/browser/b...
content/browser/browser_plugin/browser_plugin_embedder.cc:32:
seen_drag_source_ended_at_(false),
On 2014/08/21 23:36:31, Fady Samuel wrote:
> Can we do this with one flag?

Done.

[Fady Samuel, 2014-08-22 00:27:23]

lgtm

[lazyboy, 2014-08-22 00:54:20]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-08-22 00:59:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/lazyboy@chromium.org/499493002/40001

[commit-bot: I haz the power, 2014-08-22 02:50:53]

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

[commit-bot: I haz the power, 2014-08-22 07:35:22]

Committed patchset #3 (40001) as 291356