Tidy up ScriptLoader (MIME) type matching.

MIME is case-insensitively handled within Blink, so remove some
unnecessary normalization of MIME type (and "language=") strings
in ScriptLoader.

R=
BUG=

Review-Url: https://codereview.chromium.org/2497873002
Cr-Commit-Position: refs/heads/master@{#432162}
3 files changed