Extensions: kWebAppFileHandlers key now available on stable

We make the kWebAppFileHandlers key available on stable rather than
trunk.

The ExtensionFromWebApp unit tests now use a test fixture that sets up
a testing environment (by inheriting from ExtensionServiceTestBase).

This is a speculative fix for a bug (crbug.com/1062239) that seems to
have been introduced by a change to ConvertWebAppToExtension
(crrev.com/c/2087327).

The test WebAppFileHandlersAreCorrectlyConverted is re-enabled.

This CL replaces
https://chromium-review.googlesource.com/c/chromium/src/+/2108208

Bug: 1062239
Change-Id: I107a0731ab5b17faa51660384c10cab400e3cb7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120676
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753544}

extensions/common/api/_manifest_features.json

@@ -359,7 +359,7 @@
     }
   ],
   "web_app_file_handlers": {
-    "channel": "trunk",
+    "channel": "stable",
     "extension_types": ["hosted_app"]
   },
   "webview": {

extensions/common/manifest_handlers/web_app_file_handler_manifest_unittest.cc

@@ -5,9 +5,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/test/values_test_util.h"
 #include "components/services/app_service/public/cpp/file_handler.h"
-#include "components/version_info/version_info.h"
 #include "extensions/common/extension.h"
-#include "extensions/common/features/feature_channel.h"
 #include "extensions/common/manifest.h"
 #include "extensions/common/manifest_constants.h"
 #include "extensions/common/manifest_handlers/web_app_file_handler.h"
@@ -23,12 +21,6 @@ namespace errors = manifest_errors;
 namespace {
 
 class WebAppFileHandlersManifestTest : public ManifestTest {
- public:
-  // version_info::Channel::UNKNOWN needs to be specified here as long as the
-  // "web_app_file_handlers" manifest key is only enabled on trunk in
-  // extensions/common/api/_manifest_features.json.
-  WebAppFileHandlersManifestTest() : channel_(version_info::Channel::UNKNOWN) {}
-
  protected:
   ManifestData CreateManifest(const char* web_app_file_handlers) {
     const char kManifestTemplate[] =
@@ -47,9 +39,6 @@ class WebAppFileHandlersManifestTest : public ManifestTest {
         base::StringPrintf(kManifestTemplate, web_app_file_handlers));
     return ManifestData(std::move(manifest), "test");
   }
-
- private:
-  ScopedCurrentChannel channel_;
 };
 
 }  // namespace