Add JSON parser to Source/platform

Add JSON parser to Source/platform

This is based on the inspector protocol parser, with a few changes:
  - Reads a WTF::String (either 8 or 16-bit)
  - Outputs blink::JSONValues
  - Uses RefPtr<>s for compatibility with blink::JSONValue, rather than
    std::unique_ptr<>

These changes bring the parser almost exactly to the state that the
inspector parser was in, in Feb 2016, except for not supporting the
non-standard "\x" escape character.

BUG=

[iclelland, 2016-08-02 17:47:45]

The CQ bit was checked by iclelland@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-08-02 17:48:30]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-02 19:58:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-08-02 19:58:26]

Dry run: This issue passed the CQ dry run.

[iclelland, 2016-08-04 14:45:05]

The CQ bit was checked by iclelland@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-08-04 14:45:20]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-04 15:36:16]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-08-04 15:36:17]

Dry run: Try jobs failed on following builders:
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)