This cleans up the API provided by RemoveSwitchFallThrough.h, and adds
documentation about caveats of RemoveSwitchFallThrough. This change is
pure refactoring without any functional changes.

BUG=angleproject:2177
TEST=angle_end2end_tests

Change-Id: I2646e4fe3b53130b07977823cb1344e5096f67e4
Reviewed-on: https://chromium-review.googlesource.com/709194Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>