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With current implementation of devtool_tracing_handler, it doesn't do 
anything with invalid options and just use RECORD_UNTIL_FULL by default. So
the change to telemetry's tracing backend will not break backward
compatibility for browsers that don't support "record-as-much-as-possible" option.

However, this is a hacky way to maintain backward compatibility, and we 
should fix devtool_tracing_handler so it can respond with error message on
Tracing.start if there is options parsing error.

BUG=396081

Review URL: https://codereview.chromium.org/440903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288047 0039d316-1c4b-4281-b951-d872f2087c98