This patch moves the following parsing methods to ParserBase:

- ParseStatementList
- ParseStatementListItem
- ParseStatement
- ParseSubStatement (subsumed in ParseStatement)
- ParseStatementAsUnlabeled

It also refactors the Target and TargetScope objects, used by the
parser.

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

Review-Url: https://codereview.chromium.org/2307073002
Cr-Commit-Position: refs/heads/master@{#39167}