Turn on the picture runtime flag by default

Following the discussion on the "intent to ship" thread (https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/t3KRycUCGA8/VTJMRx20XiwJ ), this CL turns the flag on by default.

BUG=368830

Review URL: https://codereview.chromium.org/401403003

git-svn-id: svn://svn.chromium.org/blink/trunk@178754 bbb929c8-8fbe-4397-9dbb-9b2b20218538