Navigation Transitions: added "NavigationTransitions" runtime flag.
Navigation Transitions: added "NavigationTransitions" runtime flag.

This adds a flag to the runtime enabled flags to toggle navigation transitions.

Navigation Transitions: 

Design doc: https://docs.google.com/a/chromium.org/document/d/17jg1RRL3RI969cLwbKBIcoGDsPwqaEdBxafGNYGwiY4/edit# 
Implementation details: https://docs.google.com/a/chromium.org/document/d/1kREPtFJaeLoDKwrfmrYTD7DHCdxX1RzFBga2gNY8lyE/edit#heading=h.bng2kpmyvxq5 
BUG=370696

Review URL: https://codereview.chromium.org/287433003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@174330 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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