These comparisons are only meant to be done by AstValueFactory itself (in
its string_table_ operations), so make the Compare() function a private
member of AstValueFactory.

All other clients of AstRawStrings should compare them by pointer value.
There were only two clients which failed to abide by this rule, one
recently-added (in ModuleDescriptor) and the other in Literal::Match
(in ast.cc, added in https://code.google.com/p/v8/source/detail?r=24396).

Review URL: https://codereview.chromium.org/1069423003

Cr-Commit-Position: refs/heads/master@{#27715}