Move CSSParser inspector hooks out into their own file

The inspector hooks into CSS parsing and tokenization
via CSSParser::SourceDataHandler (now CSSParserObserver)
and CSSParser::ErrorType (now CSSParserError).

Much of this (at least the error bits) appears to actually
be dead code.

But I'm moving it out into its own file, so that I can
share this interface between the current Bison-based CSSParser
and the new CSS3 Syntax Spec hand-rolled parser I'm writing.

This came up as part of identifying CSSParser API in:
https://codereview.chromium.org/121673002/
and was identified as a layering violation there as
the inspector had to depend on BisonCSSParser.h directly.
After this patch, that layering violation is gone.

BUG=330389

Review URL: https://codereview.chromium.org/111533004

git-svn-id: svn://svn.chromium.org/blink/trunk@164365 bbb929c8-8fbe-4397-9dbb-9b2b20218538