Reenable some BrowserTests on FYI bots

Some of the BrowserTest suite was flaky, and disabled. The various sources of
the flakes have been since addressed. I'm turning them back on the FYI bot to
verify.

TBR=sky@chromium.org
TEST=BrowserTest in mash_browser_tests

Bug: 707195
Change-Id: I36f3bf2c98135842a68217b6bf2982065f9dec3b
Reviewed-on: https://chromium-review.googlesource.com/574613
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487472}

testing/buildbot/filters/mojo.fyi.browser_tests.filter

+BrowserTest.*
+
 # Failing test
 -BrowserTest.FullscreenBookmarkBar
 
-# Flaky tests -crbug.com/707195
--BrowserTest.AboutVersion
--BrowserTest.CanDuplicateTab
--BrowserTest.ClearPendingOnFailUnlessNT
--BrowserTest.DefaultMediaDevices
--BrowserTest.GetSizeForNewRenderView
--BrowserTest.JavascriptAlertActivatesTab
--BrowserTest.Title
-
 # Trying to whitelist
 BrowserDialogTest.Invoke
 CastSessionBrowserTest.CreateAndDestroy