Remove Widget::clipRectChanged notification

There is no reason for this notification to exist. The only client of this
notification is WebPluginContainerImpl, and that class just uses it to send a
general notification that will be sent during widgetPositionsUpdated anyway.

This CL is an incremental step towards avoiding computing the widget's clip
rect during layout because the clip rect depends on compositing information and
therefor creates a chicken/egg situation.

R=eseidel@chromium.org
BUG=339898

Review URL: https://codereview.chromium.org/154083009

git-svn-id: svn://svn.chromium.org/blink/trunk@166993 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Disable unreliable test parts.

Initiator for images loaded from style recalculation is unreliable now.

BUG=155260, 306228
NOTRY=true
TBR=vsevik

Review URL: https://codereview.chromium.org/153043007

git-svn-id: svn://svn.chromium.org/blink/trunk@166994 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Refactor CSSDefaultStyleSheets from a statics-only class into a singleton.

It used to host statics of primitive pointer type but for OilPan we need those statics to become persistent roots and thus we can't simply have them in static variables anymore.

This CL also cleans up some FIXMEs related to CSSDefaultStyleSheets object: loading of default styles is moved into constructor which is invoked lazily when instance is accessed for the first time and loadDefaultStylesheetIfNecessary is killed.

BUG=

Review URL: https://codereview.chromium.org/133083005

git-svn-id: svn://svn.chromium.org/blink/trunk@166995 bbb929c8-8fbe-4397-9dbb-9b2b20218538