[Chromecast] Use channel-associated MediaLoadDeferrer interface

This CL replaces uses of MediaLoadDeferrer interface with
channel-associated methods. This is used to avoid race conditions
in Chromecast builds because we want to make sure MediaLoad state
is updated before any media element is run.

BUG=internal b/74121154
TEST= media_load_deferrer interface connect correctly

Change-Id: Icc69e62f8fea0b13a2d79f0c52428b480d171823
Reviewed-on: https://chromium-review.googlesource.com/1070492
Commit-Queue: Jiawei Li <lijiawei@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562503}

• chromecast/renderer/DEPS[diff]
• chromecast/renderer/cast_media_load_deferrer.cc[diff]
• chromecast/renderer/cast_media_load_deferrer.h[diff]