Exclude zip-file loading from "Load Unpacked Extension"

This was only implemented half-way, and the file picker still limited
choices to directories.  So this code only got run (incorrectly)
by selecting a folder that ends in .zip (which happens on ChromeOS
for a mounted zip archive).

BUG=418753

Review URL: https://codereview.chromium.org/676503003

Cr-Commit-Position: refs/heads/master@{#301141}

chrome/browser/ui/webui/extensions/extension_loader_handler.cc

@@ -219,27 +219,14 @@ void ExtensionLoaderHandler::HandleDisplayFailures(
 
 void ExtensionLoaderHandler::LoadUnpackedExtensionImpl(
     const base::FilePath& file_path) {
-  if (EndsWith(file_path.AsUTF16Unsafe(),
-               base::ASCIIToUTF16(".zip"),
-               false /* case insensitive */)) {
-    scoped_refptr<ZipFileInstaller> installer = ZipFileInstaller::Create(
-        ExtensionSystem::Get(profile_)->extension_service());
-
-    // We do our own error handling, so we don't want a load failure to trigger
-    // a dialog.
-    installer->set_be_noisy_on_failure(false);
-
-    installer->LoadFromZipFile(file_path);
-  } else {
-    scoped_refptr<UnpackedInstaller> installer = UnpackedInstaller::Create(
-        ExtensionSystem::Get(profile_)->extension_service());
-
-    // We do our own error handling, so we don't want a load failure to trigger
-    // a dialog.
-    installer->set_be_noisy_on_failure(false);
-
-    installer->Load(file_path);
-  }
+  scoped_refptr<UnpackedInstaller> installer = UnpackedInstaller::Create(
+      ExtensionSystem::Get(profile_)->extension_service());
+
+  // We do our own error handling, so we don't want a load failure to trigger
+  // a dialog.
+  installer->set_be_noisy_on_failure(false);
+
+  installer->Load(file_path);
 }
 
 void ExtensionLoaderHandler::OnLoadFailure(