GitLab

This CL adds a telemetry benchmark to use gmail page and uses the
scrollbar and mouss drag action to scroll the page. The test calculates
the mid-point of scrollbar and drags it till the end of the page.

BUG=464685

Review URL: https://codereview.chromium.org/985603003

Cr-Commit-Position: refs/heads/master@{#319629}