Navigation transitions: Expose navigation transitions feature to WebRuntimeFeatures.

Thought it would be nice to keep this feature behind the experimental flag, but be able to turn it on via a finch trial from chrome.

Will be followed by: https://codereview.chromium.org/389583004/

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/386943009

git-svn-id: svn://svn.chromium.org/blink/trunk@178575 bbb929c8-8fbe-4397-9dbb-9b2b20218538
WebAudio: Add SSE2 optimization for DirectConvolver

Benchmark result from
http://rtoy.github.io/webaudio-hacks/tests/time-convolver-short.html,
running Linux on a Z620:

M36: 1680 ms
Opt: 1060 ms

Time is now reduced by about 33%.

For Windows running on a Z600:
M38: 3005 ms
Opt: 2280 ms

A savings of about 25%.

BUG=395276
TESTS=convolution-mono-mono still passes.

Review URL: https://codereview.chromium.org/408563003

git-svn-id: svn://svn.chromium.org/blink/trunk@178576 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move fitBorderToLinesIfNeeded and most of deleteLineBoxTree into RenderBlockFlow.

fitBorderToLinesIfNeeded can only be called on block flows, so it can be moved
altogether.

deleteLineBoxTree can have its logic moved, leaving (for now) only an ASSERT
declaring that it wouldn't have done anything anyway for now.

BUG=393945, 302024

Review URL: https://codereview.chromium.org/404993002

git-svn-id: svn://svn.chromium.org/blink/trunk@178577 bbb929c8-8fbe-4397-9dbb-9b2b20218538