[Autoplay] Add block autoplay pref

Add a block autoplay pref that is available on all
platforms except Android.

BUG=865548

Change-Id: I4b09105da1ed901ac8a8ad8bb9ec984237b0662b
Reviewed-on: https://chromium-review.googlesource.com/1165494Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581296}

chrome/common/pref_names.cc

@@ -2603,6 +2603,9 @@ const char kAutoplayAllowed[] = "media.autoplay_allowed";
 
 // Holds URL patterns that specify URLs that will be allowed to autoplay.
 const char kAutoplayWhitelist[] = "media.autoplay_whitelist";
+
+// Boolean that specifies whether autoplay blocking is enabled.
+const char kBlockAutoplayEnabled[] = "media.block_autoplay";
 #endif  // !defined(OS_ANDROID)
 
 // Integer that holds the value of the next persistent notification ID to be

chrome/common/pref_names.h

@@ -915,6 +915,7 @@ extern const char kWebDriverOverridesIncompatiblePolicies[];
 #if !defined(OS_ANDROID)
 extern const char kAutoplayAllowed[];
 extern const char kAutoplayWhitelist[];
+extern const char kBlockAutoplayEnabled[];
 #endif
 
 extern const char kNotificationNextPersistentId[];