RFC: Make BisonCSSParser purely stack allocated.

I was looking at how to port the BisonCSSParser and CSSParserObserver to the oilpan (GC) heap and noticed that we had a pointer from the BisonCSSParser back to the stack allocated CSSParserObserver/StyleSheetHandler. While it worked correctly making the BisonCSSParser stack allocated will IMO simplify the lifetime reasoning and make it less likely that we introduce errors.
This comes at the "expense" of always allocating a comment parser in the StyleSheetHandler. It is only used in the Inspector code so I am hoping that is okay.

R=ager@chromium.org, erik.corry@gmail.com, eseidel@chromium.org, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/216433003

git-svn-id: svn://svn.chromium.org/blink/trunk@170530 bbb929c8-8fbe-4397-9dbb-9b2b20218538