Add field trial for the WebAPK server URL

chrome/common/chrome_features.h

Index: chrome/common/chrome_features.h
diff --git a/chrome/common/chrome_features.h b/chrome/common/chrome_features.h
index a24f65f3f533258d6d43869172b5723387ad7b70..c1c4d309f81d4e5f0b78d72d400a8e9dc9ac37c0 100644
--- a/chrome/common/chrome_features.h
+++ b/chrome/common/chrome_features.h
@@ -97,6 +97,10 @@ extern const base::Feature kQuickUnlockPin;
 extern const base::Feature kEHVInputOnImeMenu;
 #endif  // defined(OS_CHROMEOS)
 
+#if defined(OS_ANDROID)

[dominickn, 2016/10/24 00:32:37]

Nit: probably should call this kWebApks.

+extern const base::Feature kWebApk;

[dominickn, 2016/10/24 00:32:37]

If you're adding this, you should look at revamping the existing kill-switch and
enabling flag for WebAPKs. You can enable / disable WebAPKs with
--enable-features=WebAPKs or --disable-features=WebAPKs, and you can use that as
a kill switch in Finch as well. That makes the existing enabling flag and kill
switch a bit redundant.

if (base::FeatureList::IsEnabled(kWebApks)) will check whether the feature is on
or off, so that should suffice for enabling and killing it

On the command line, you can pass parameters like this:

 --enable-features="WebApks<FieldTrialName"
--force-fieldtrials=FieldTrialName/GroupName
--force-fieldtrial-params=FieldTrialName.GroupName:server_url/server_url_goes_here.

(You'll need to URL escape the URL in the parameter, but I think that's the only
gotcha).

Then you can remove the server URL command line switch entirely, and just query
the variations like you have, because that command line will populate the
variation correctly.

I had a look at ChromeWebApkHost, and I think that the native side should take
care of querying the feature / etc., whilst the Java side will persist whatever
the value is in prefs afterwards.

Thoughts? I know you've got a bunch on right now, but this will greatly tidy up
the command line flag / Finch situation for WebAPKs. It'll remove a Finch kill
switch and the current server URL command line flag in favour of just using the
Feature architecture, so you don't have this duplication. E.g. the semantics of
what happens when you have the Finch feature on, but the kill switch on and vice
versa are confusing. Unifying under a single Feature with parameters makes it
much tidier.

+#endif
+
 // DON'T ADD RANDOM STUFF HERE. Put it in the main section above in
 // alphabetical order, or in one of the ifdefs (also in order in each section).