Add support for `width` based Width Client Hint

LayoutTests/http/tests/misc/client-hints-accept.php

Index: LayoutTests/http/tests/misc/client-hints-accept.php
diff --git a/LayoutTests/http/tests/misc/client-hints-accept.php b/LayoutTests/http/tests/misc/client-hints-accept.php
index df31db30f6db1b2393124f72e3bb07ffb7f25218..89a28731af16ecdc0092d0085290b8a4aaff1330 100644
--- a/LayoutTests/http/tests/misc/client-hints-accept.php
+++ b/LayoutTests/http/tests/misc/client-hints-accept.php
@@ -14,6 +14,7 @@
         var loadRWImage = function() {
             var img = new Image();
             img.src = 'resources/image-checks-for-width.php';
+            img.width = '500';

[Mike West, 2015/06/18 06:20:03]

Can you help me understand how this is testing the new behavior?

[Yoav Weiss, 2015/06/18 06:30:21]

With the new behavior, the `Width:` hint is not sent unless there are either
`width` or `sizes` attributes. Adding `width` here makes sure that the hint is
sent.

[Mike West, 2015/06/18 06:34:35]

Got it, thank you.

             img.onload = t.step_func(function(){ t.done(); });
             img.onerror = t.step_func(unreached);
             document.body.appendChild(img);