A new V8 API object v8::TracingCpuProfiler is introduced.
Client can create it on an isolate to enable JS CPU profiles collected
during tracing session.

Once the v8.cpu_profile2 tracing category is enabled the profiler emits
CpuProfile and CpuProfileChunk events with the profile data.

BUG=chromium:406277

Review-Url: https://codereview.chromium.org/2396733002
Cr-Commit-Position: refs/heads/master@{#40054}