TimelineFlameChart: provide separate view for another thread.

Timeline has data about at least two threads.
FlameChart looks like broken if we mix both threads in one view.
So we splitted it and the top view shows the events from the main thread.
The bottom view shows events from gpu thread.

The screenshot has been attached to the bug.

BTW: I forgot to remove defaultColorGenerator implementation in the previous patch.
BUG=316167

Review URL: https://codereview.chromium.org/148343007

git-svn-id: svn://svn.chromium.org/blink/trunk@166902 bbb929c8-8fbe-4397-9dbb-9b2b20218538