Rename LinearTimingFunction::preset to shared

Rename for consistency with other singletons in the codebase.

BUG=350998

Review URL: https://codereview.chromium.org/196383011

git-svn-id: svn://svn.chromium.org/blink/trunk@172822 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove calls to testRunner testRepaint/repaintSweep*

These functions are incompatible with force compositing mode and are of
dubious value.

Also, consolidated all of the shadow-multiple tests into a single test
as they are now identical.  The fast/repaint/box-shadow-* files
are attached to an open bug, so leaving to make it easier to
investigate.

BUG=366438

Review URL: https://codereview.chromium.org/251433002

git-svn-id: svn://svn.chromium.org/blink/trunk@172823 bbb929c8-8fbe-4397-9dbb-9b2b20218538