Don't create an SkString in DiscardablePixelRef::isDiscardable.

strcmp should be equivalent to creating an SkString and then comparing.

Resolves a FIXME warning about the thread safety of DEFINE_STATIC_LOCAL.

BUG=375263

Review URL: https://codereview.chromium.org/295463002

git-svn-id: svn://svn.chromium.org/blink/trunk@174898 bbb929c8-8fbe-4397-9dbb-9b2b20218538