[tracing] Switch TracedValue to pickles: lighter and accountable

[tracing] Switch TracedValue to pickles: lighter and accountable

TraceValue is used pretty much everywhere in tracing.
It is conceptually similar but exposes a much simpler API of
base::Value. Until now it used base::Value under the hoods.

Problems:
  - We need a way to account accurately the memory used by tracing for
    the sake of memory profiling (See crbug.com/495628).
  - base::Value is too complex / uses too many std::strings
    (e.g., tracing doesn't need keys to be strings, just static char*)
  - is very hard to predict memory usage of std::string (strongly
    impl dependant)

Proposed solution:
  This CL introduces a drop-in replacement for TracedValue, inspired by
  the following principles:
   - Be simple and small: just a pickle.
   - Don't copy strings if not necessary: keep the const char* ptrs to
     "quoted" static strings.

For more details see the attached bug and the design doc goo.gl/xw7tyR

BUG=495628

Review URL: https://codereview.chromium.org/1173793003

Cr-Commit-Position: refs/heads/master@{#333932}