This patch moves the following parsing method to ParserBase:

- ParseSwitchStatement

It also removes ParseCaseClause and merges it with ParseSwitchStatement,
mainly to avoid the complexity of introducing one more abstract typedef
to be shared between parser implementations, but also because the merged
ParseSwitchStatement is now only 59 lines.

R=adamk@chromium.org, marja@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2324843005
Cr-Commit-Position: refs/heads/master@{#39337}