GitLab

This was added with the intention of using Size as a vector, replacing use of
IntSize. Since we have Vector2d now, negative sizes should not exist, so clamp
them in set_width/set_height and the constructor.

Not covered by tests, as we can't test DCHECKs.

TBR=sky
BUG=160158
Relanding: https://codereview.chromium.org/11365160/

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=167255

Review URL: https://chromiumcodereview.appspot.com/11410024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175535 0039d316-1c4b-4281-b951-d872f2087c98