[Sensors] Add an UMA histogram for Generic Sensor function calls

All Sensor functions have a histogram value that is recorded when the function is called.

Specification: https://w3c.github.io/sensors/
BUG=606766

Review-Url: https://codereview.chromium.org/2476253002
Cr-Commit-Position: refs/heads/master@{#431259}
3 files changed