DevTools: promisify ContentProvider.requestContent and all its clients.

third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js

Index: third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js
diff --git a/third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js b/third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js
index b13cba16ae6aa0fe394a2587c4ddd6e24ac00e85..27a87fafa85ea8082e5cafd6d861ed2ad046024e 100644
--- a/third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js
+++ b/third_party/WebKit/Source/devtools/front_end/elements/StylesSidebarPane.js
@@ -459,15 +459,16 @@ WebInspector.StylesSidebarPane.prototype = {
     {
         if (!styleSheetHeader)
             return;
-        styleSheetHeader.requestContent(onStyleSheetContent.bind(this, styleSheetHeader.id));
+        styleSheetHeader.requestContent().then(onStyleSheetContent.bind(this, styleSheetHeader.id));
 
         /**
          * @param {string} styleSheetId
-         * @param {string} text
+         * @param {?string} text
          * @this {WebInspector.StylesSidebarPane}
          */
         function onStyleSheetContent(styleSheetId, text)
         {
+            text = text || "";
             var lines = text.split("\n");
             var range = WebInspector.TextRange.createFromLocation(lines.length - 1, lines[lines.length - 1].length);
             this._addBlankSection(this._sectionBlocks[0].sections[0], styleSheetId, range);