Add a blink feature flag to control web app URL handling features.

design doc: https://docs.google.com/document/d/19dGklalQTRtRrG3PKrVbDRmYPLHGLnsGEwUaed7sAFQ/edit?usp=sharing

design overview cl: https://chromium-review.googlesource.com/c/chromium/src/+/2393252

Chrome status: https://www.chromestatus.com/feature/5739732661174272

Bug: 1072058
Change-Id: I6ef280af95514e0ac1abfacdcf6b113ee7b7af57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444439
Commit-Queue: Lu Huang <luhua@microsoft.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814308}

third_party/blink/common/features.cc

@@ -757,5 +757,11 @@ const base::Feature kPreferCompositingToLCDText = {
 const base::Feature kLogUnexpectedIPCPostedToBackForwardCachedDocuments{
     "LogUnexpectedIPCPostedToBackForwardCachedDocuments",
     base::FEATURE_ENABLED_BY_DEFAULT};
+
+// Controls URL handling feature in web apps. Controls parsing of "url_handlers"
+// field in web app manifests. See explainer for more information:
+// https://github.com/WICG/pwa-url-handler/blob/master/explainer.md
+const base::Feature kWebAppEnableUrlHandlers{"WebAppEnableUrlHandlers",
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -309,6 +309,8 @@ BLINK_COMMON_EXPORT extern const base::Feature kPreferCompositingToLCDText;
 
 BLINK_COMMON_EXPORT extern const base::Feature
     kLogUnexpectedIPCPostedToBackForwardCachedDocuments;
+
+BLINK_COMMON_EXPORT extern const base::Feature kWebAppEnableUrlHandlers;
 }  // namespace features
 }  // namespace blink