Reland of Changed CSSParserContext argument from * to & for null safety.

Reland of Changed CSSParserContext argument from * to & for null safety. (patchset #1 id:1 of https://codereview.chromium.org/2801893003/ )

Reason for revert:
This patch was reverted for breaking tests when it was landed at the same time as a clashing patch https://codereview.chromium.org/2783543002/. Both of these patches were reverted.

Landing them one at a time and rebasing should solve the problem.

Original issue's description:
> Revert of Changed CSSParserContext argument from * to & for null safety. (patchset #4 id:60001 of https://codereview.chromium.org/2786153004/ )
> 
> Reason for revert:
> 
> Findit identified CL at revision 462380 as the culprit for
> failures in the build cycles as shown on:
> https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzkxMmU1Zjc4N2VlNDE1Zjg3NDkyOGI5Njg0ZWE1MGRjNzVmODVkNDAM
> 
> Original issue's description:
> > Changed CSSParserContext argument from * to & for null safety.
> > 
> > Changed the CSSParserContext argument to the parseSingleValue method in
> > the property APIs from a pointer to a reference to ensure that nullptr
> > cannot be passed.
> > 
> > Note that this patch changes the syntax inside parseSingleValue
> > implementations to get the address of CSSParserContext references to pass to 
> > utility methods that still expect a pointer. Future patches should change these 
> > utility methods to also take references instead of pointers, at which point the 
> > syntax inside parseSingleValue implementations can be changed back to pass the 
> > object.
> > 
> > This patch:
> > - changed the CSSParserContext parameter to parseSingleValue in
> >   CSSPropertyAPIMethods.json5 so that the generated API header files (including
> >   the base class header file) would update 
> > - changed the parameter in each of the .cpp files for the property APIs in
> >   core/css/properties
> > - updated the single call site in CSSPropertyParser.cpp
> > - updated the implementations of parseSingleValue in the property APIs to use
> >   reference syntax
> > 
> > Diff of generated files:
> > https://gist.github.com/BugsNash/1850f85afc7313dd3dac1a890299ad07/revisions
> > 
> > BUG=668012
> > 
> > Review-Url: https://codereview.chromium.org/2786153004
> > Cr-Commit-Position: refs/heads/master@{#462380}
> > Committed: https://chromium.googlesource.com/chromium/src/+/912e5f787ee415f874928b9684ea50dc75f85d40
> 
> TBR=shend@chromium.org,meade@chromium.org,bugsnash@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=668012
> 
> Review-Url: https://codereview.chromium.org/2801893003
> Cr-Commit-Position: refs/heads/master@{#462383}
> Committed: https://chromium.googlesource.com/chromium/src/+/b7aae4a78d2902cb83671f7ef7792b14180ee462

TBR=meade@chromium.org,lukasza@google.com,jkrcal@google.com,foolip@google.com,ellyjones@google.com,shend@chromium.org,findit-for-me@appspot.gserviceaccount.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=668012

[Bugs Nash, 2017-04-06 23:53:45]

Created Reland of Changed CSSParserContext argument from * to & for null safety.