Makes the following changes to the automated VR latency testing script:
- Adds the functionality to save latency and correlation results in a format
    compatible with the Chrome performance dashboard
- Refactors the code into separate classes and files for improved readability
    and to better support future Windows latency testing
- Runs the test multiple times (default 10) to get an average and standard deviation

BUG=708747

Review-Url: https://codereview.chromium.org/2823883003
Cr-Commit-Position: refs/heads/master@{#466147}