[Chromecast] Update extension request handler to load files too

If we recieve a request for an extension that doesn't have a 'cast_url'
key present defer to the normal extension request processor to load
the files from disk

This is also required to load background pages (as it is from disk)

Bug: b/73768170
Test: Load extension that loads from disk, verify fallback occurs
Change-Id: I7bf477f92dbebfd37077ed417e12d17a03d35308
Reviewed-on: https://chromium-review.googlesource.com/934924Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553682}

chromecast/browser/extension_request_protocol_handler.cc

@@ -4,6 +4,7 @@
 
 #include "chromecast/browser/extension_request_protocol_handler.h"
 
+#include "extensions/browser/extension_protocols.h"
 #include "extensions/browser/extension_system.h"
 #include "extensions/browser/info_map.h"
 #include "extensions/common/extension.h"
@@ -265,6 +266,8 @@ net::URLRequestJob* ExtensionRequestProtocolHandler::MaybeCreateJob(
     // This can't be done in the constructor as extensions::ExtensionSystem::Get
     // will fail until after this is constructed.
     info_map_ = extensions::ExtensionSystem::Get(browser_context_)->info_map();
+    default_handler_ = extensions::CreateExtensionProtocolHandler(
+        false, const_cast<extensions::InfoMap*>(info_map_));
   }
 
   const extensions::Extension* extension =
@@ -277,8 +280,8 @@ net::URLRequestJob* ExtensionRequestProtocolHandler::MaybeCreateJob(
 
   std::string cast_url;
   if (!extension->manifest()->GetString("cast_url", &cast_url)) {
-    LOG(ERROR) << "No 'cast_url' value for extension";
-    return nullptr;
+    // Defer to the default handler to load from disk.
+    return default_handler_->MaybeCreateJob(request, network_delegate);
   }
 
   // Replace chrome-extension://<id> with whatever the extension wants to go to.

chromecast/browser/extension_request_protocol_handler.h

@@ -31,6 +31,8 @@ class ExtensionRequestProtocolHandler
  private:
   content::BrowserContext* const browser_context_;
   mutable const extensions::InfoMap* info_map_ = nullptr;
+  mutable std::unique_ptr<net::URLRequestJobFactory::ProtocolHandler>
+      default_handler_;
 };
 
 }  // namespace chromecast