Move the compositing reason descriptions of the header

Previously, this array was in the header, where it was copied into every
compilation unit. Now we use the linker.

Review URL: https://codereview.chromium.org/378603002

git-svn-id: svn://svn.chromium.org/blink/trunk@177609 bbb929c8-8fbe-4397-9dbb-9b2b20218538