This patch removes TracingCategoryObserver API and moves the creation of
observer inside platform initialization, by assuming that either
Platform::AddTraceStateObserver is implemented correctly to add observer to
tracing controller that implemented by embedders, or default tracing controller
has already been set up and attached to platform before
v8::V8::InitializePlatform is called.

BUG=v8:5590

Review-Url: https://codereview.chromium.org/2471583004
Cr-Commit-Position: refs/heads/master@{#40739}