Deduplicate some chrome/browser/android constants and helpers

In jumbo builds many cc files compile in the same translation
unit and then they share the same anonymous namespace. Having
the same constants and helper functions in several files will
then result in compilation failures.

In preparation for jumbo in chrome/browser, this patch
deduplicates the GetWebContents() helper function and the constants
kOMADrmMessageMimeType, kOMADrmContentMimeType,
kOMADrmRightsMimeType1, kOMADrmRightsMimeType2.

Bug: 746957
Change-Id: Ie268f77c7a099a8de01ae08c70870306f38fc05a
Reviewed-on: https://chromium-review.googlesource.com/897612
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#534182}
5 files changed