Enable back/forward mouse button propagation to javascript.

Enable the ExtendedMouseButtons feature. Approved Intent to Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/MrKpBV26ik4/D_zmoAJrAQAJ

BUG=680741

Change-Id: I146a535b1eb8b502e43681bc1fbb83d595f29ed8
Reviewed-on: https://chromium-review.googlesource.com/994673Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548089}

content/public/common/content_features.cc

@@ -107,7 +107,7 @@ const base::Feature kExpensiveBackgroundTimerThrottling{
 
 // Enables exposing back/forward mouse buttons to the renderer and the web.
 const base::Feature kExtendedMouseButtons{"ExtendedMouseButtons",
-                                          base::FEATURE_DISABLED_BY_DEFAULT};
+                                          base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables a blink::FontCache optimization that reuses a font to serve different
 // size of font.

third_party/WebKit/LayoutTests/TestExpectations

@@ -3347,9 +3347,6 @@ crbug.com/825262 [ Win7 ] external/wpt/css/css-transitions/properties-value-impl
 crbug.com/825733 [ Win7 ] media/color-profile-video-seek-filter.html [ Pass Timeout Failure ]
 crbug.com/754986 media/video-transformed.html [ Pass Failure ]
 
-# Intent to Ship pending for this test
-crbug.com/680741 external/wpt/uievents/mouse/mouse_buttons_back_forward-manual.html [ Skip ]
-
 # Sheriff 2018-03-29
 crbug.com/827088 bluetooth/requestDevice/chooser/new-scan-device-added.html [ Pass Crash ]
 crbug.com/827209 fast/events/middleClickAutoscroll-latching.html [ Pass Failure ]