Use feature architecture as kill switch for WebAPKs instead of field trial

chrome/browser/android/chrome_feature_list.cc

Index: chrome/browser/android/chrome_feature_list.cc
diff --git a/chrome/browser/android/chrome_feature_list.cc b/chrome/browser/android/chrome_feature_list.cc
index 0f9705fff70df981df9e574da2e46e6981fdf8a7..b8ef37901fe19da01ddd75880def384b7dbdb83b 100644
--- a/chrome/browser/android/chrome_feature_list.cc
+++ b/chrome/browser/android/chrome_feature_list.cc
@@ -51,6 +51,7 @@ const base::Feature* kFeaturesExposedToJava[] = {
     &kSpecialLocaleFeature,
     &kSpecialLocaleWrapper,
     &kTabReparenting,
+    &kWebApks,
     &kCCTExternalLinkHandling,
     &offline_pages::kOfflinePagesBackgroundLoadingFeature,
     &offline_pages::kBackgroundLoaderForDownloadsFeature,
@@ -103,6 +104,12 @@ const base::Feature kCCTExternalLinkHandling{"CCTExternalLinkHandling",
 const base::Feature kUserMediaScreenCapturing{
     "UserMediaScreenCapturing", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Makes "Add to Home screen" in the app menu generate an APK for the shortcut
+// URL which opens Chrome in fullscreen.
+// For WebAPKs to be enabled, the WebAPK feature must be enabled and the
+// --enable-webapk command line flag must be set.

[pkotwicz, 2016/10/24 04:24:28]

The reason that I added the comment about the --enable-webapk command line flag
is to explain the default value. It is a bit confusing that the feature is
"default-on" even though it is not launched

[dominickn, 2016/10/24 04:35:47]

Oh, I didn't notice that ENABLED_BY_DEFAULT (features almost always land
DISABLED_BY_DEFAULT if they're under development). My instinct is that it really
should be DISABLED_BY_DEFAULT since it isn't launched and is still under very
heavy development (it's very rare for a feature to land ENABLED_BY_DEFAULT).
Then the flag to turn on WebAPKs is just --enable-features=WebApks, and if you
really want you can then make the --enable-webapk flag enable the feature as
well.

Again, this will consolidate everything for WebAPKs under this one feature,
which is desirable. The kill switch via Finch should still take precedence if
necessary.

[pkotwicz, 2016/10/24 15:33:26]

I have disabled the feature by default as you have requested.

I am keeping the current logic of requiring both the feature to be enabled and
the command line switch. We will probably "enable the feature by default" prior
to "enabling the command line switch by default" because we will want there to
be a way for random developers to try WebAPKs out

[dominickn, 2016/10/25 00:13:32]

Random developers can try it out by passing --enable-features=WebApks on the
command line - that's what I meant by the Feature API being able to fully
replace the command line string too. :)

+const base::Feature kWebApks{"WebApks", base::FEATURE_ENABLED_BY_DEFAULT};
+
 static jboolean IsEnabled(JNIEnv* env,
                           const JavaParamRef<jclass>& clazz,
                           const JavaParamRef<jstring>& jfeature_name) {