This patch implements TracingCategoryObserver to set global flag when a V8
specific category is enabled. Previously, we set a global flag each time when we
encounter a top level trace event, and use it as a global check. With this
patch, we can set a group of flags when tracing is enabled; besides, we make
V8 tracing feature use V8 flags instead of defining its own flag in a messy way.

With this patch, whatever V8 flag we want to imply in tracing, we define another
integer flag, and the original V8 flag will set it to 0x01 when passing by
commandline, tracing will set it to 0x10 when we start tracing and reset the bit
when we stop tracing.

Review-Url: https://codereview.chromium.org/2436273002
Cr-Commit-Position: refs/heads/master@{#40659}