Add WebView Tracing API.

Add classes/interfaces necessary for the support of tracing API
in WebView.

In particular:
- TracingController: singleton class for starting and stopping
  tracing and inspecting tracing status.
- TracingConfig: holds tracing configuration, such as categories
  to filter and the tracing mode.
- TracingOutputStream: interface for capturing tracing data.
- TracingFileOutputStream: supporting class for capturing tracing
  data to a file.

BUG: 63750258
Test: in progress

Change-Id: I7decd486788766fc8dfa766ae027c02f9d4c6f80