Reset fontselector() when removing stylesheets and left stylesheets still have @font-face rules.

If not reset, CSSSegmentedFontFaceCache has mulitple fontFaces for the same @font-face rule.
This causes fast/css/font-face-html-as-svg flaky (Timeout in mac-blink-rel).

BUG=305885,309370

Review URL: https://codereview.chromium.org/73993002

git-svn-id: svn://svn.chromium.org/blink/trunk@162432 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce UndoStack class

This patch introduces UndoStack class moved from EditorClientImpl class to make Blink API layer thin for ease of maintenance.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/74753003

git-svn-id: svn://svn.chromium.org/blink/trunk@162433 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use WTF::bind instead of ExecutionContextTask in a bunch of places

This is now possible due to https://src.chromium.org/viewvc/blink?view=rev&revision=162388,
which added support for posting a Closure to ExecutionContext.

Also cleanup a bit of code, removing an unneeded static_cast in ExecutionContext.cpp
and an unused Task class in modules/webdatabase.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/80013002

git-svn-id: svn://svn.chromium.org/blink/trunk@162434 bbb929c8-8fbe-4397-9dbb-9b2b20218538