Revert 107563 - Fix up compositor mocking.

Revert 107563 - Fix up compositor mocking.

Two changes:
a) detangle the mock compositor from compositor_test_support
a) pull in compositor when using a mock compositor

Rationale:

a) this target was meant to support using use_webkit_compositor=1 when WK isn't initialized; sometimes we will want to use the mock compositor when WK is initialized

b) we can imagine scenarios where we want to switch between mock and real compositors in the same executable, and it's one less target to update

BUG=101477
TEST=bots stay green

Review URL: http://codereview.chromium.org/8395043

TBR=backer@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=107564