Added three new pyauto performance tests for login to Gmail/Calendar/Docs.
Added three new pyauto performance tests for login to Gmail/Calendar/Docs.

Also added some error-checking to 2 existing tests to ensure data files
exist before use, and removed the functionality for doing a single trial
run before the multiple iterations in the tab performance tests (the
single trial run information was not being used).

BUG=chromium-os:18185
TEST=None

Review URL: http://codereview.chromium.org/7795012

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@99003 0039d316-1c4b-4281-b951-d872f2087c98

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