Enable the sizes attribute and srcset 'w' descriptor by default

Following the intent to ship thread[1], this CL enables the sizes attribute and srcset's 'w' descriptor by default.

BUG=357586

[1] https://groups.google.com/a/chromium.org/d/msg/blink-dev/GRfwz951FHo/RUue8U49iuwJ

Review URL: https://codereview.chromium.org/356833007

git-svn-id: svn://svn.chromium.org/blink/trunk@176988 bbb929c8-8fbe-4397-9dbb-9b2b20218538