Add JSON serializers and file storage to memory_inspector.

This CL introduces the data serializers that allow:
1) To exchange JSON objects with the (upcoming) HTML UI.
2) To store/retrieve from disk memory dumps.

BUG=340294
NOTRY=true

Review URL: https://codereview.chromium.org/178693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254451 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254451