MIME type predicates and case-insensitive matching.

The registered type portion of a MIME type must be compared
case-insensitively for equality. Make all Blink-side predicates (e.g.,
is*MIMEType()) do such equality testing.

If the equality testing delegates to external platform predicates,
then ensure that the MIME type is first translated to lower case.
(This is due to the net/base/ MIME type predicates assuming
lower-cased input.)

The downstream benefit of this is that we can let go of various
explicit lower case conversions before invoking these predicates.

R=jochen
BUG=

Review URL: https://codereview.chromium.org/122743002

git-svn-id: svn://svn.chromium.org/blink/trunk@164453 bbb929c8-8fbe-4397-9dbb-9b2b20218538