Update testSetCustomChromeFlags to only append to chrome flags, not override
Update testSetCustomChromeFlags to only append to chrome flags, not override

This is in keeping with the spirit of the ExtraChromeFlags() method. Whenever
it's overridden, it should only ever append to the list of chrome flags,
never completely override it.

BUG=None
TEST=None

R=dennisjeffrey@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10041001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@131597 0039d316-1c4b-4281-b951-d872f2087c98

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