Use Document::isActive() instead of Document::renderer()

A bunch of random DOM code was poking at the render tree via
Document::renderer() when what it really wants to know is whether the document
is active. This CL removes some of these DOM -> RenderTree dependencies in
favor of Document::isActive(), which is tied into the Document's lifecycle.

R=esprehn
BUG=299011

Review URL: https://codereview.chromium.org/25504007

git-svn-id: svn://svn.chromium.org/blink/trunk@158675 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add minimal redundant media source seek layout test

Adds media source layout test to verify the Chrome for Android redundant seek suppression optimization fix in r226166 works as expected.

BUG=267111
R=acolwell@chromium.org
TEST=New layout test passes on ToT desktop, Chrome for Android, and fails with timeout on Chrome for Android with r226166 reverted (expected due to ChunkDemuxer pending StartWaitingForSeek+seek to ephemeral time that was optimized away prior to r226166).

Review URL: https://codereview.chromium.org/25560004

git-svn-id: svn://svn.chromium.org/blink/trunk@158676 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move LayoutTestSupport.cpp/.h to Source/platform/

BUG=297477

Review URL: https://codereview.chromium.org/25426003

git-svn-id: svn://svn.chromium.org/blink/trunk@158677 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move EventTracer.cpp/.h to Source/platform/

BUG=297477

Review URL: https://codereview.chromium.org/25440002

git-svn-id: svn://svn.chromium.org/blink/trunk@158678 bbb929c8-8fbe-4397-9dbb-9b2b20218538