Mousedown on an <a> element should focus on an outer focusable element.

This is a regression by Blink r155083.

Blink doesn't focus on <a> elements by mousedown historically.
r155083 changed EventHandler so that it search ancestors for an isFocusable
element to fix a problem of clicking on already-focused <a>. However if <a>
inside a focusable element is clicked, the <a> was isFocusable and we stopped to
search for outer elements on it.

We should continue to search for an isMouseFocusable element even if <a>
is found, and check already-focused earlier.

* EventHandler::dispatchMouseEvent:
 - We should check already-focused in the ancestor traversal, and should check
   isMouseFocusable instead of isFocusable.
 - |elementIsMouseFocusable| is now equivalent to |!!element|

[1] http://src.chromium.org/viewvc/blink?view=rev&revision=155083

BUG=304909
TEST=automated

Review URL: https://codereview.chromium.org/26441004

git-svn-id: svn://svn.chromium.org/blink/trunk@159149 bbb929c8-8fbe-4397-9dbb-9b2b20218538