Focus plugin on gesturetapdown.

Focus plugin on gesturetapdown.

We focus the plugin on mousedown/touchstart, so similarly we should
focus it on gesturetapdown.

This fixes IME input on plugins, if a plugin gets gesturetapdown
without any mousedown/touchstart, the plugin will be focused correctly
and input fed to the plugin would work properly.
This could happen if the plugin doesn't have any touch handlers, then
tapping sends gesture event(s) to plugin, without any touchstart.

BUG=383484
Test=Using chrome app webview, which is a plugin.
Enable virutal keyboard in chrome://flags if it's not enabled.
In chromebook pixel: load the sample
browser app
https://github.com/GoogleChrome/chrome-app-samples/tree/master/webview-samples/browser
Navigate to a simple page that has <input> element, e.g.
http://jsbin.com/togif/1/
Now without generating any mouse/touchpad inside the content area, tap on the screen on the input box in the jsbin page.
Notice virtual keyboard opening and it is possible to type stuffs into the input box.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176152

[lazyboy, 2014-06-13 00:06:46]

+Kevin for initial review.

[kevers, 2014-06-13 17:47:56]

lgtm

[lazyboy, 2014-06-13 17:56:37]

+Adam for OWNERS review.

[abarth-chromium, 2014-06-13 21:50:39]

It looks like we already focus the plugin on touchstart.  Is there some way to
get a GestureTap without a touchstart?  In the case of a touch-driven gesture,
does this mean we'll focus the plugin twice?

[lazyboy, 2014-06-14 01:18:30]

On 2014/06/13 21:50:39, abarth wrote:
> It looks like we already focus the plugin on touchstart.  Is there some way to
> get a GestureTap without a touchstart?  In the case of a touch-driven gesture,
> does this mean we'll focus the plugin twice?

If the plugin has touch handlers then it gets touchstart (and no gesture
events),
and if it doesn't than it gets gesturetap/gesturetapdown (and no touch events).

After talking to sadrul@, I found we should be using gesturetapdown instead
of gesturetap, as touch scrolling only sends gesturetapdown, not gesturetap.

Patch Set #3.

[abarth-chromium, 2014-06-14 03:19:56]

lgtm

[lazyboy, 2014-06-14 04:00:44]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-06-14 04:01:05]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/lazyboy@chromium.org/333813002/40001

[commit-bot: I haz the power, 2014-06-14 05:02:25]

Change committed as 176152