DevTools: Timeline: prepare for making timeline filters optional.

Timeline has hardcoded set of filters.
We are going to support flame chart which has no filters at the first glance.
So we should be ready for this.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/101653003

git-svn-id: svn://svn.chromium.org/blink/trunk@163436 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use gin::IsolateHolder as one and only holder of v8::Isolates

BUG=321631
R=marja@chromium.org

Review URL: https://codereview.chromium.org/110153002

git-svn-id: svn://svn.chromium.org/blink/trunk@163437 bbb929c8-8fbe-4397-9dbb-9b2b20218538