Fix issues around unhandled values of MediaStreamRequestResult

When preparing to extend MediaStreamRequestResult with a distinct value
for explicit user rejection, multiple issues were discovered where:
- Switch-cases neglected to handle some enum values.
- Switch-cases handled NUM_MEDIA_REQUEST_RESULTS as though it were a
  valid value.
- The analogous enum for histograms was not updated with some
  recently added values.
- In some cases, all enum values were actually handled, but the use of
  `default` risked similar issues in the future.

Before extending MediaStreamRequestResult, we take a moment to fix these
pre-existing issues.

Bug: 416454904
Change-Id: I7d458d4c24a2cc07a8379e101a72b691d4a7060e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6529964
Reviewed-by: Tove Petersson <tovep@chromium.org>
Reviewed-by: Palak Agarwal <agpalak@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459292}