This patch const-qualifies some methods of ParserBase.
It also unqualifies some methods of Parser and Preparser.
The reason for the latter is that, in principle, the methods
of AstNodeFactory should be allowed to change the factory's
state, therefore should not be const and should not be used
from const-qualified parser/pre-parser methods.

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

Review-Url: https://codereview.chromium.org/2263973003
Cr-Commit-Position: refs/heads/master@{#38813}