[Switch Access] Fix reference to menu_panel.html

A recent change to how the Switch Access extension is loaded caused the
Switch Access menu and Switch Access back button (both rendered in the
SwitchAccessPanel) to stop showing up. This change fixes the string
referencing menu_panel.html to match the new relative path.

TBR=dtseng@chromium.org

Bug: None
Change-Id: I64d39e623c7d81ecf197ab9b5c35b83c2cecfd3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079269Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#745662}

chrome/browser/chromeos/accessibility/switch_access_panel.cc

@@ -19,7 +19,7 @@ const int kFocusRingBuffer = 5;
 const std::string& UrlForContent() {
   static const base::NoDestructor<std::string> url(
       std::string(EXTENSION_PREFIX) + extension_misc::kSwitchAccessExtensionId +
-      "/menu_panel.html");
+      "/switch_access/menu_panel.html");
   return *url;
 }