Add support for `width` based Width Client Hint

LayoutTests/http/tests/misc/client-hints-accept-meta.html

Index: LayoutTests/http/tests/misc/client-hints-accept-meta.html
diff --git a/LayoutTests/http/tests/misc/client-hints-accept-meta.html b/LayoutTests/http/tests/misc/client-hints-accept-meta.html
index b710aeddb195833887a26636fa936d442f1e132a..d463e57f7cf8e118caa9b6087af1b7b7d858418e 100644
--- a/LayoutTests/http/tests/misc/client-hints-accept-meta.html
+++ b/LayoutTests/http/tests/misc/client-hints-accept-meta.html
@@ -12,6 +12,7 @@
         var loadRWImage = function() {
             var img = new Image();
             img.src = 'resources/image-checks-for-width.php';
+            img.sizes = '100vw';

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

Here too.

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

Same. Meant to make sure the hint is sent.

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