Chrome talks to Play to install WebAPKs.

chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java

Index: chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java b/chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java
index f63bca998a36140a33c3bb0e7e489fb1f95923c3..d0b817afde96ee964e5021153481574a372a4ee4 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/DeferredStartupHandler.java
@@ -46,6 +46,7 @@ import org.chromium.chrome.browser.preferences.ChromePreferenceManager;
 import org.chromium.chrome.browser.preferences.privacy.PrivacyPreferencesManager;
 import org.chromium.chrome.browser.share.ShareHelper;
 import org.chromium.chrome.browser.webapps.ChromeWebApkHost;
+import org.chromium.chrome.browser.webapps.WebApkInstaller;
 import org.chromium.chrome.browser.webapps.WebApkVersionManager;
 import org.chromium.chrome.browser.webapps.WebappRegistry;
 import org.chromium.components.minidump_uploader.CrashFileManager;
@@ -270,6 +271,11 @@ public class DeferredStartupHandler {
                     PrecacheLauncher.updatePrecachingEnabled(mAppContext);
 
                     if (ChromeWebApkHost.isEnabled()) {
+                        if (ChromeWebApkHost.canUseGooglePlayToInstallWebApk()) {
+                            ChromeApplication application = (ChromeApplication) mAppContext;

[dominickn, 2016/12/08 03:31:26]

mAppContext is just the result of calling ContextUtils.getApplicationContext(),
right? If so, maybe you don't need to initialize this here, and you can just
fetch the PlayInstaller off the application context in WebAplInstaller when you
need it?

[Xi Han, 2016/12/08 18:03:07]

Yes, we could move it back to the WebApkInstaller. The only reason that it is
here is: we want to initialize the delegate only once. This delegates binds to
the Play install service, and we want to reuse the existing connection whenever
it is possible. So we only keep one instance of the delegate in the
WebApkInstaller.

Additionally, sometimes the delegate is null (e.g. user use chrome_public_apk
which doesn't have clank code), so we don't know whether it has been initialized
by checking whether the object is null. We could have another static flag set in
the WebApkInstaller, and that was what I did before. I am not sure which way is
better: whether initialing it here, or in WebApkInstaller with a static flag to
indicate whether it is has been initialized or not. Any suggestion? 

[dominickn, 2016/12/12 03:15:24]

The delegate lives on the application context, so it's only initialised once.
One reason to not do something here is that the deferred startup handler might
not run tasks at all. Most of the stuff in here is cache or data clearing that
doesn't matter if it doesn't run. However, initialising the Play install
delegate must happen reliably.

How expensive is it to repeatedly construct the Play install delegate? If you
really want to only create it once, then the right thing to do is to make it a
singleton. I suggest:

 * renaming the createGooglePlayWebApkInstallDelegate() method to
getGooglePlayWebApkInstallDelegate()
 * making PlayInstallWebApkClient a singleton using the Holder idiom (see
PermissionDialogController for an example)
 * making getGooglePlayWebApkInstallDelegate() inside ChromeApplicationInternal
simply return PlayInstallWebApkClient.getInstance(), which will lazily
initialise the singleton (once) if you use the Holder pattern

Then getGooglePlayWebApkInstallDelegate() is cheap and initialises the delegate
once only. Hence WebApkInstaller can just have the delegate as a member variable
and fetch it in its constructor since the singleton-ness is handled where it
should be - on the singleton class.

Does that make sense?

[Xi Han, 2016/12/12 19:09:53]

I see, yes, we shouldn't trust  DeferredStartupHandler to complete all the
required work. Thanks for all the suggestions. I adopted the singleton pattern
since it is the right way to do.

+                            WebApkInstaller.setGooglePlayWebApkInstallDelegate(
+                                    application.createGooglePlayWebApkInstallDelegate());
+                        }
                         WebApkVersionManager.updateWebApksIfNeeded();
                     }