Handle dpad inputs for Starboard.

This allows the TV remote to be used to control cast content.

The logic in this CL is slightly different from the code checked into
1.56core. The main differences are:
* kSbKeyReturn is mapped to ui::DomCode::ENTER instead of
  ui::DomCode::SELECT
* CastRuntimeContentBrowserClientStarboard creates a WebContentsObserver
  to focus the WebContents (instead of adding a new focus call in
  CastWebContentsImpl, since that would affect non-starboard cast
  devices)
* I've reworked the code to not use build flags (except in a BUILD.gn
  file to determine whether starboard or a dummy is used)

Test:
Cast to a third-party TV and gLinux. There is a bug on gLinux if you
initiate a new cast session while casting, resulting in a black screen.
This occurs even without this CL, and does not seem to repro on 3P TVs.

Bug: b:362992995
Change-Id: I5718aea70e15616ad222d300a424c48ca31bb48e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5834127
Reviewed-by: Shawn Quereshi <shawnq@google.com>
Reviewed-by: Vigen Issahhanjan <vigeni@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Antonio Rivera <antoniori@google.com>
Cr-Commit-Position: refs/heads/main@{#1351735}