Bug 1289634 - Disable Widevine CDM by default on Linux, but prompt user to enable and download on first use. r=gerald

We don't want the Widevine CDM to be downloaded by default on Linux, as
the CDM is proprietary software and the user hasn't opted in to having such
software on their system. By leaving the CDM visible but disabled, we'll
prompt the user the first time EME is used, and they can approve the
download.

This means the Widevine CDM won't be downloaded by default. The user will
need to opt-in to enablding DRM playback before we'll download proprietary
CDMs.


MozReview-Commit-ID: GLBoK2Czjcc

--HG--
extra : rebase_source : 35aac5a84a1b779149d08fe5a2c85179bd00756d

browser/app/profile/firefox.js

@@ -1350,8 +1350,17 @@ pref("media.gmp-eme-adobe.enabled", true);
 
 #ifdef MOZ_WIDEVINE_EME
 pref("media.gmp-widevinecdm.visible", true);
+// On Linux Widevine is visible but disabled by default. This is because
+// enabling Widevine downloads a proprietary binary, which users on an open
+// source operating system didn't opt into. The first time a site using EME
+// is encountered, the user will be prompted to enable EME, whereupon the
+// EME plugin binary will be downloaded if permission is granted.
+#ifdef XP_LINUX
+pref("media.gmp-widevinecdm.enabled", false);
+#else
 pref("media.gmp-widevinecdm.enabled", true);
 #endif
+#endif
 
 // Play with different values of the decay time and get telemetry,
 // 0 means to randomize (and persist) the experiment value in users' profiles,