mac: move browser_tests back to pool:Chrome at 50% experiment.

Will promote back to non-experimental once pool:Chrome-quarantine has
been folded back into pool:Chrome.

Bug: 828031
Change-Id: Icb4f7da5b0aab18ec951bb612acf8d3e7e155a27
Reviewed-on: https://chromium-review.googlesource.com/c/1302657
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603184}
2 files changed