Increase Mac browser_tests experiment to 50%.

Bug: 828031
Change-Id: I395c4b33b7ac394645bf626c00185b4f9efcc289
Reviewed-on: https://chromium-review.googlesource.com/c/1294309
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601928}
2 files changed