Add DecryptingRendererFactory

Returning a DecryptingRenderer from the MojoRendererFactory introduced
some regressions:
- DecryptingRenderer crashes when used with URL media resources.
- MediaPlayerRendererClient sets up a delayed crash due to a static_cast
  into MojoRenderer*.

This CL moves the creation of DecryptingRenderer into its own factory.
The DecryptingRendererFactory is a small wrapper around
MojoRendererFactory.
This gets rid of both regressions, and makes it explicit where/when
DecryptingRenderers are used.

Bug: 919494, 919819
Change-Id: I52a091a13dd858ccb787e17fe1abacbb32ec3916
Reviewed-on: https://chromium-review.googlesource.com/c/1405814
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622166}
9 files changed