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Previously, the options for tracing were passed around with different ad hoc
systems. Strings in some places, base::debug::TraceOptions enum in others,
and a content::TraceOptions in yet another. There were two separate ad-hoc
string formats. Similar messes were present with category filters: sometimes
we passed strings, sometimes the CategoryFilter.

This patch though enormous looking simply consolidates all this ad-hockery
into base::debug::TraceOptions. It may look like the call sites have gotten
more verbose,
but the end result of this is a consistent understanding of TraceOptions.

There is one exception to this consolidation: devtools has to maintain its own
mapping of string->TraceOptions encoding. This is because DevTools makes
guarantees about backward compatibility: if base changes its mind about the
string form of a TraceOption, devtools needs to keep supporting the old form.

BUG=396081
TBR=cevans@chromium.org

Review URL: https://codereview.chromium.org/425593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287348 0039d316-1c4b-4281-b951-d872f2087c98