Introduce UndoStack::didUnloadFrame() for replacement of UndoStack::clearUndoRedoOperations()

This patch introduces UndoStack::didUnloadFrame() for replacement of UndoStack::clearUndoRedoOperations() to remove undo steps belong to unloading frame rather than removing all undo steps in undo/redo stacks. This patch also removed redundant function Editor::clearUndoRedoOperations().

BUG=280923
TEST=LayoutTests/editing/undo/undo-after-removing-iframe.html

Review URL: https://codereview.chromium.org/82973002

git-svn-id: svn://svn.chromium.org/blink/trunk@162817 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add test for Transform operations that depend on boxSize are not candidates for the compositor.

Tim Loh added the code to perform this check in r162453.

BUG=321840

Review URL: https://codereview.chromium.org/85123002

git-svn-id: svn://svn.chromium.org/blink/trunk@162818 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce throttling in FileReader.

Thanks to this CL we can increase the count of the file read request in the test case.
That was reduced in https://codereview.chromium.org/84693006/ because of crash bug.

BUG=295428

Review URL: https://codereview.chromium.org/86523004

git-svn-id: svn://svn.chromium.org/blink/trunk@162819 bbb929c8-8fbe-4397-9dbb-9b2b20218538