Added platform/TracedValue.{h,cpp}

These are counterparts of base::debug::TracedValue that implement
blink's ConvertableToTraceFormat and are produced from JSONValue.

BUG=361045

Review URL: https://codereview.chromium.org/245143002

git-svn-id: svn://svn.chromium.org/blink/trunk@172036 bbb929c8-8fbe-4397-9dbb-9b2b20218538