Add blink::Platform methods needed by PeerConnectionDependencyFactory (1/2)

... when peer_connection_dependency_factory.cc|h gets Onion soup'ed.

Namely, the methods are:

- GetWebRTCMediaPermission()
- GetWebRTCRendererPreferences()

In order to test the code being added right away, the CL also
routes the callsite through the newly added methods.

The CL eliminates the needs for
PeerConnectionDependencyFactory::CreatePortAllocator(), to reference
RenderFrameImpl and RenderViewImpl directly.

In a follow up CL, further methods will be added to replace the remaining
references to base::CommandLine & switches.

BUG=787452
R=guidou@chromium.org, haraken@chromium.org

Change-Id: Ia897d304a21395c89fb032a4bfc80ec1a82ff858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799102
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696097}
4 files changed