Add perf_expectations/ and empty expectations JSON.
Add perf_expectations/ and empty expectations JSON.

Adding the directory and empty file now so I can
continue making perf-related changes to the buildbot
infrastructure.  The expectations file will get filled
in with some initial values after the infrastructure
changes have been made.

BUG=18579
TEST=none

Review URL: http://codereview.chromium.org/178024

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@24817 0039d316-1c4b-4281-b951-d872f2087c98

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