Bug 1503653 - Part 2. Enable WebP by default. r=tnikkel

Bug 1249474 suggested that we add image/webp to the front of the Accept
header for images, to indicate to servers that we actually support WebP.

Differential Revision: https://phabricator.services.mozilla.com/D8120

image/test/mochitest/bug496292-1.sjs

@@ -9,7 +9,7 @@ function handleRequest(request, response)
   file.append("test");
   file.append("mochitest");
 
-  if (request.getHeader("Accept") == "*/*") {
+  if (request.getHeader("Accept") == "image/webp,*/*") {
     file.append('blue.png');
   } else {
     file.append('red.png');

modules/libpref/init/all.js

@@ -4707,7 +4707,7 @@ pref("image.decode-immediately.enabled", false);
 pref("image.downscale-during-decode.enabled", true);
 
 // The default Accept header sent for images loaded over HTTP(S)
-pref("image.http.accept", "*/*");
+pref("image.http.accept", "image/webp,*/*");
 
 // The threshold for inferring that changes to an <img> element's |src|
 // attribute by JavaScript represent an animation, in milliseconds. If the |src|
@@ -4790,7 +4790,7 @@ pref("image.multithreaded_decoding.limit", -1);
 pref("image.multithreaded_decoding.idle_timeout", 600000);
 
 // Whether we attempt to decode WebP images or not.
-pref("image.webp.enabled", false);
+pref("image.webp.enabled", true);
 
 // Limit for the canvas image cache. 0 means we don't limit the size of the
 // cache.