Reland "Clear highlight when the highlighted node gets removed from the document"

Reland "Clear highlight when the highlighted node gets removed from the document"

Some web pages use a mask element to intercept tap gesture and then hide
the element immediately. The orphaned highlight can be very annoying.
For example, pages that show a popup often use a full screen transparent
element to intercept events, and will hide the popup if the user attempt
to tap on the main document. If we don't remove highlight from the hidden
element, the whole screen will be covered by a highlight.

This patch clears highlight from a layer when the tap node is no longer valid.

TEST=webkit_unit_tests:LinkHighlightTest.resetDuringNodeRemoval
BUG=232926
R=jamesr@chromium.org

Originally reviewed in https://chromiumcodereview.appspot.com/14582012
Later reverted in https://codereview.chromium.org/14921004/
The test failure was due to failing to unregister mocked URLs after test.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=149838

[trchen, 2013-05-07 01:15:31]

This patch is identical to the original patch except appending
Platform::current()->unitTestSupport()->unregisterAllMockedURLs() to the end of
both tests.

[jamesr, 2013-05-07 01:41:50]

lgtm

[commit-bot: I haz the power, 2013-05-07 01:42:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/trchen@chromium.org/14729006/1

[commit-bot: I haz the power, 2013-05-07 02:27:14]

Change committed as 149838