This enforces that we use ParseExpressionCoverGrammar, and not ParseExpression,
for several tricky cases. Also clarify comment on ParserBase::ParseExpression().

Change-Id: I1d1289abdf539c96f4b42f97c79a8adf3c06e728
Reviewed-on: https://chromium-review.googlesource.com/783171
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49647}