Bug 979555: Add default value for layers.offmainthreadcomposition.log-animations preference to all.js. r=dzbarsky

It's good practice to have default values for all preferences in all.js.
Otherwise some preference APIs throw exceptions and/or leave values
uninitialized.  It also makes the preference show up in about:config.

Bug 875204 accidentally removed this line from all.js even though the
pref was not removed.  (The pref was initially added in bug 780342.)

modules/libpref/src/init/all.js

@@ -3976,6 +3976,9 @@ pref("layers.offmainthreadcomposition.force-basic", false);
 // Whether to animate simple opacity and transforms on the compositor
 pref("layers.offmainthreadcomposition.async-animations", false);
 
+// Whether to log information about off main thread animations to stderr
+pref("layers.offmainthreadcomposition.log-animations", false);
+
 pref("layers.bufferrotation.enabled", true);
 
 pref("layers.componentalpha.enabled", true);