History: Add aria-label attribute to the button which removes an entry.

On mobile, the button for removing a history entry has no textual label,
only an "X" image. Add an aria-label attribute to the button elements to
improve accessibility.

BUG=285367
TEST=See repro steps in bug.

Review URL: https://codereview.chromium.org/34413011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230410 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230410