DevTools: Timeline: cosmetic changes.

Minor style changes in _createFilters method.

New method _selectPresentationMode was introduced.
If the requested mode doesn't exist then it selects Events mode.

BUG=none
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/93763003

git-svn-id: svn://svn.chromium.org/blink/trunk@163530 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use a gin::ContextHolder to hold the main blink v8::Context

BUG=321631
R=marja@chromium.org

Review URL: https://codereview.chromium.org/110223002

git-svn-id: svn://svn.chromium.org/blink/trunk@163531 bbb929c8-8fbe-4397-9dbb-9b2b20218538