DevTools: do not assign empty content to CSS UISourceCode in case of protocol error

third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js

Index: third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
index 25421fcfd9d585865a6d8e0982eb98e5f7f86814..70ea3fa5ec41318bb14f2727f0f149489fe09b9f 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/CSSModel.js
@@ -52,7 +52,7 @@ SDK.CSSModel = class extends SDK.SDKModel {
     /** @type {!Map.<string, !Object.<!Protocol.Page.FrameId, !Array.<!Protocol.CSS.StyleSheetId>>>} */
     this._styleSheetIdsForURL = new Map();
 
-    /** @type {!Map.<!SDK.CSSStyleSheetHeader, !Promise<string>>} */
+    /** @type {!Map.<!SDK.CSSStyleSheetHeader, !Promise<?string>>} */
     this._originalStyleSheetText = new Map();
 
     /** @type {!Multimap<string, !Protocol.CSS.StyleSheetId>} */
@@ -784,12 +784,12 @@ SDK.CSSModel = class extends SDK.SDKModel {
 
   /**
    * @param {!Protocol.CSS.StyleSheetId} styleSheetId
-   * @return {!Promise<string>}
+   * @return {!Promise<?string>}
    */
   _ensureOriginalStyleSheetText(styleSheetId) {
     var header = this.styleSheetHeaderForId(styleSheetId);
     if (!header)
-      return Promise.resolve('');
+      return Promise.resolve(/** @type {?string} */ (null));
     var promise = this._originalStyleSheetText.get(header);
     if (!promise) {
       promise = this.getStyleSheetText(header.id);
@@ -807,7 +807,7 @@ SDK.CSSModel = class extends SDK.SDKModel {
 
   /**
    * @param {!SDK.CSSStyleSheetHeader} header
-   * @return {!Promise<string>}
+   * @return {!Promise<?string>}
    */
   originalStyleSheetText(header) {
     return this._ensureOriginalStyleSheetText(header.id);
@@ -909,24 +909,23 @@ SDK.CSSModel = class extends SDK.SDKModel {
 
   /**
    * @param {!Protocol.CSS.StyleSheetId} styleSheetId
-   * @return {!Promise<string>}
+   * @return {!Promise<?string>}
    */
   getStyleSheetText(styleSheetId) {
     /**
      * @param {?Protocol.Error} error
      * @param {?string} text
-     * @return {string}
+     * @return {?string}
      */
     function textCallback(error, text) {
       if (error || text === null) {
         console.error('Failed to get text for stylesheet ' + styleSheetId + ': ' + error);
-        text = '';
-        // Fall through.
+        return null;
       }
       return SDK.CSSModel.trimSourceURL(text);
     }
 
-    return this._agent.getStyleSheetText(styleSheetId, textCallback).catchException(/** @type {string} */ (''));
+    return this._agent.getStyleSheetText(styleSheetId, textCallback).catchException(/** @type {?string} */ (null));
   }
 
   _resetStyleSheets() {