media: Set MediaClientAndroid in the GPU process.

Now on Android we use MediaDrmBridge in the GPU process. We should set
MediaClientAndroid the same as we do in the Browser process.

Summary of changes:
- Add ContentClient::GetMediaClientAndroid() API.
- SetMediaClientAndroid() in GPU process.

Today in the browser, we call media::SetMediaClientAndroid() directly
from content embedders (e.g. ChromeBrowserMainPartsAndroid), which
seems hacky. We should use ContentClient::GetMediaClientAndroid() in
the browser process as well so that content will talk to the embedder,
and then talk to media. I'll work on it after this CL.

TBR=slan@chromium.org
BUG=605330
TEST=Encrypted Widevine contents plays in the GPU process on Android K.

Review-Url: https://codereview.chromium.org/1907263003
Cr-Commit-Position: refs/heads/master@{#390517}
(cherry picked from commit f258baab74abe43f7f77bdae8b74c61c07f95f51)

Review URL: https://codereview.chromium.org/1934193003 .

Cr-Commit-Position: refs/branch-heads/2704@{#336}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
25 files changed