Revert the patch that has disabled Panels for M53 (https://codereview.chromium.org/2083113002)

The current plan is to remove the code for M54 but
keep the Panels enabled for M53 to provide more time for transition.

BUG=571511

Review-Url: https://codereview.chromium.org/2176823002
Cr-Commit-Position: refs/heads/master@{#407262}

chrome/app/generated_resources.grd

@@ -5558,7 +5558,7 @@ Keep your key file in a safe place. You will need it to create new versions of y
         Panels
       </message>
       <message name="IDS_FLAGS_PANELS_DESCRIPTION" desc="Description for the flag to enable Panel windows.">
-        Allow extensions to create panel windows that open outside of the browser frame. By default or if disabled, an attempt to open a panel will open a popup instead. Panels on desktop OSes are deprecated and will be removed soon.
+        Allow extensions to create panel windows that open outside of the browser frame. Attempts to open a panel will open a popup instead if not enabled. Default behavior is to allow only for whitelisted extensions. Enabled behavior is to allow for all extensions. Disabled behavior is to disallow panels for any extension.
       </message>
       <message name="IDS_FLAGS_WEBGL2_NAME" desc="Name of the 'Enable WebGL 2.0' flag.">
         WebGL 2.0 Prototype

chrome/browser/ui/panels/panel_manager.cc

@@ -131,16 +131,11 @@ bool PanelManager::ShouldUsePanels(const std::string& extension_id) {
   }
 #endif  // USE_X11 && !OS_CHROMEOS
 
-// Panels are an Eraser target. See http://crbug.com/571511 for details.
-// TODO(dimich): remove the rest of Panels code (except ChromeOS) once
-// erasure is stable.
-#if defined(OS_CHROMEOS)
-  // Without --enable-panels, only support Hangouts on ChromeOS.
+  // Without --enable-panels, only support Hangouts.
   for (const char* id : extension_misc::kHangoutsExtensionIds) {
     if (extension_id == id)
       return true;
   }
-#endif  // OS_CHROMEOS
 
   return false;
 }