Bug 1373780 - Part 1: Added smiley.png.headers. r=annevk

For web-platform-tests imageset.https.sub,html [1], it will fetch smiley.png
and use window.performace to query the download size to determine the image is
actually downloaded or not. [2]

However this test is a mixed-content-blocked test, (https: document to
load some images from http:), and performace API only allows pages with
different origin to use this API if the HTTP header 'Timing-Allow-Origin' is
specified. [3]

So we added a smiley.png.header to specifiy 'Timing-Allow-Origin'.

[1]: http://searchfox.org/mozilla-central/rev/b258e6864ee3e809d40982bc5d0d5aff66a20780/testing/web-platform/tests/mixed-content/imageset.https.sub.html
[2]: http://searchfox.org/mozilla-central/rev/b258e6864ee3e809d40982bc5d0d5aff66a20780/testing/web-platform/tests/preload/resources/preload_helper.js#13
[3]: https://www.w3.org/TR/resource-timing-1/#timing-allow-origin

testing/web-platform/meta/MANIFEST.json

@@ -298559,6 +298559,11 @@
      {}
     ]
    ],
+   "images/smiley.png.headers": [
+    [
+     {}
+    ]
+   ],
    "images/threecolors.png": [
     [
      {}
@@ -597436,6 +597441,10 @@
    "44481e3f374d75b735ac01ae83dc8ed9330238fc",
    "support"
   ],
+  "images/smiley.png.headers": [
+   "4f956a2d550f09444239d1c0bc640200a3c60102",
+   "support"
+  ],
   "images/threecolors.png": [
    "3c394b0d754a7b02514b46d5483a0014d5dcdca6",
    "support"

testing/web-platform/tests/images/smiley.png.headers

@@ -0,0 +1 @@
+Timing-Allow-Origin: *