Fix WebViewTest.TestContextMenu on official builders.

Fix WebViewTest.TestContextMenu on official builders.

We stopped showing "Inspect Element" on dev/beta/stable, therefore
TestContextMenu does not seem to find any menu item when right-click
is performed.
This CL makes sure we always have an <input> on the right-click target
so that we have some items to show in the context menu.

BUG=470300
Test=None, internal test only change.

Committed: https://crrev.com/2a65624aa3d60e2b5a13a8dff8862c07f2d422ad
Cr-Commit-Position: refs/heads/master@{#322326}

[lazyboy, 2015-03-26 00:40:51]

lazyboy@chromium.org changed reviewers:
+ fsamuel@chromium.org

[lazyboy, 2015-03-26 00:40:51]

Waiting for the trybots to see if this fix is enough or it has some platform
dependency...
See CL description for rationale of this change, thanks.

[Fady Samuel, 2015-03-26 02:37:35]

lgtm

[lazyboy, 2015-03-26 03:24:01]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2015-03-26 03:25:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1016033010/40001

[commit-bot: I haz the power, 2015-03-26 05:23:04]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-03-26 05:23:33]

Patchset 3 (id:??) landed as
https://crrev.com/2a65624aa3d60e2b5a13a8dff8862c07f2d422ad
Cr-Commit-Position: refs/heads/master@{#322326}