[Telemetry] Fix page_cycler on windows.
[Telemetry] Fix page_cycler on windows.

1. You can't del a key in itervalues(). Have to use value(). Previously this
   raised a RuntimeError "dictionary changed size during iteration."
2. On win, the Gpu key is an empty dict, so we have to allow for that when
   pulling the IO stats.

BUG=None
TEST=moz page cycler on win
NOTRY=True
TBR=ccameron@chromium.org

Review URL: https://chromiumcodereview.appspot.com/14615013

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@200034 0039d316-1c4b-4281-b951-d872f2087c98

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