[Android Refactor] Reland: Merge WebappDataStorage#LAST_USED_UNSET and LAST_USED_INVALID

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java

Index: chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
index 1997ce97f279cb9b1ced3daafedd907400a81a4b..b05ebdcbdf2a63686a4bb0f6b904d1ab698ba2e2 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java
@@ -112,13 +112,19 @@ public class WebappRegistry {
             protected final WebappDataStorage doInBackground(Void... nothing) {
                 // Create the WebappDataStorage on the background thread, as this must create and
                 // open a new SharedPreferences.
-                return WebappDataStorage.open(webappId);
+                WebappDataStorage storage = WebappDataStorage.open(webappId);
+                // The first read from {@link WebappDataStorage} must be done in the background.

[dominickn, 2017/06/13 01:36:57]

This is an interesting restriction! Is it new?

[pkotwicz, 2017/06/13 03:14:29]

Thank you for your question! It forced me to do more investigation. The strict
mode exception is thrown by SharedPreferencesImpl#awaitLoadLocked() if the
SharedPreferences have not finished loading.
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/...

We would get a strict mode exception if we tried to access SharedPreferences
immediately after the strict mode block in WebappActivity#preInflationStartup()
(I checked that this is the case)

[dominickn, 2017/06/13 23:13:21]

Perhaps this bit of code should be moved to warmUpSharedPrefs then? The whole
purpose of warming shared prefs is to avoid this issue in one go....

+                // Otherwise a strict mode exception is thrown. Do it now.
+                storage.getLastUsedTime();
+                return storage;
             }
 
             @Override
             protected final void onPostExecute(WebappDataStorage storage) {
-                // Guarantee that last used time != WebappDataStorage.LAST_USED_INVALID. Must be
-                // run on the main thread as SharedPreferences.Editor.apply() is called.
+                // Update the last used time in order to prevent
+                // {@link WebappRegistry@unregisterOldWebapps()} from deleting the
+                // WebappDataStorage. Must be run on the main thread as
+                // SharedPreferences.Editor.apply() is called.
                 mStorages.put(webappId, storage);
                 mPreferences.edit().putStringSet(KEY_WEBAPP_SET, mStorages.keySet()).apply();
                 storage.updateLastUsedTime();