Clipboard: Add test helper for portable type checks

For portable types, reading from the clipboard involves both a check of
IsFormatAvailable() and ReadAvailableTypes(). This CL adds a helper
which does both of these checks, simplifying the code and reducing the
chances that new code will accidentally only check one of these
methods (see: https://crbug.com/1215137).

For non-portable types, comments have been left to explain why this
helper is not used.

Bug: 1241712, 1215137
Change-Id: I25f03a2d4c2559e688fbf26a06893cde4725d673
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3111332


Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/main@{#914517}