Make favicons for about:history and about:bookmarks switch from lodpi to hidpi when dragging browser

chrome/browser/resources/shared/js/util.js

Index: chrome/browser/resources/shared/js/util.js
diff --git a/chrome/browser/resources/shared/js/util.js b/chrome/browser/resources/shared/js/util.js
index 1377f40c4ff2a4f5bc90832f9e6182a6104189dd..917fa685bed641086a7888caf34e31b98cf6f520 100644
--- a/chrome/browser/resources/shared/js/util.js
+++ b/chrome/browser/resources/shared/js/util.js
@@ -38,6 +38,22 @@ function chromeSend(name, params, callbackName, callback) {
 }
 
 /**
+ * Returns the scale factors supported by this platform.
+ * @return {array} The supported scale factors.
+ */
+function getSupportedScaleFactors() {
+  var supportedScaleFactors = [];
+  if (cr.isMac || cr.isChromeOS) {
+    supportedScaleFactors.push(1);
+    supportedScaleFactors.push(2);
+  } else {
+    // Windows must be restarted to display at a different scale factor.
+    supportedScaleFactors.push(window.devicePixelRatio);
+  }
+  return supportedScaleFactors;
+}
+
+/**
  * Generates a CSS url string.
  * @param {string} s The URL to generate the CSS url for.
  * @return {string} The CSS url string.
@@ -57,6 +73,36 @@ function url(s) {
 }
 
 /**
+ * Generates a CSS -webkit-image-set for a chrome:// url.
+ * An entry in the image set is added for each of getSupportedScaleFactors().
+ * The scale-factor-specific url is generated by replacing the first instance of
+ * 'scalefactor' in |path| with the numeric scale factor.
+ * @param {string} path The URL to generate an image set for.
+ *     'scalefactor' should be a substring of |path|.
+ * @return {string} The CSS -webkit-image-set.
+ */
+function imageset(path) {
+  var supportedScaleFactors = getSupportedScaleFactors();
+
+  var replaceStartIndex = path.indexOf('scalefactor');
+  if (replaceStartIndex < 0)
+    return url(path);
+
+  var s = '';
+  for (var i = 0; i < supportedScaleFactors.length; ++i) {
+    var scaleFactor = supportedScaleFactors[i];
+    var pathWithScaleFactor = path.substr(0, replaceStartIndex) + scaleFactor +
+        path.substr(replaceStartIndex + 'scalefactor'.length);
+
+    s += url(pathWithScaleFactor) + ' ' + scaleFactor + 'x';
+
+    if (i != supportedScaleFactors.length - 1)
+      s += ', ';
+  }
+  return '-webkit-image-set(' + s + ')';
+}
+
+/**
  * Parses query parameters from Location.
  * @param {string} location The URL to generate the CSS url for.
  * @return {object} Dictionary containing name value pairs for URL
@@ -228,14 +274,34 @@ function appendParam(url, key, value) {
 }
 
 /**
- * Creates a new URL for a favicon request.
+ * Creates a CSS -webkit-image-set for a favicon request.
+ * @param {string} url The url for the favicon.
+ * @param {number=} opt_size Optional preferred size of the favicon.
+ * @param {boolean=} opt_sessionFavicon Optional flag to indicate if
+ *     requesting a session favicon.
+ * @return {string} -webkit-image-set for the favicon.
+ */
+function getFaviconImageSet(url, opt_size, opt_sessionFavicon) {
+  var size = opt_size || 16;
+  var type = opt_sessionFavicon ? 'session-favicon' : 'favicon';
+  return imageset(
+      'chrome://' + type + '/size/' + size + '@scalefactorx/' + url);
+}
+
+/**
+ * Creates a new URL for a favicon request for the current device pixel ratio.
+ * The URL must be updated when the user moves the browser to a screen with a
+ * different device pixel ratio. Use getFaviconImageSet() for the updating to
+ * occur automatically.
  * @param {string} url The url for the favicon.
  * @param {number=} opt_size Optional preferred size of the favicon.
  * @param {boolean=} opt_sessionFavicon Optional flag to indicate if
  *     requesting a session favicon.
  * @return {string} Updated URL for the favicon.
  */
-function getFaviconUrl(url, opt_size, opt_sessionFavicon) {
+function getFaviconUrlForCurrentDevicePixelRatio(url,
+                                                 opt_size,
+                                                 opt_sessionFavicon) {
   var size = opt_size || 16;
   var type = opt_sessionFavicon ? 'session-favicon' : 'favicon';
   return 'chrome://' + type + '/size/' + size + '@' +