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This CL refactors WebappRegistry and WebappDataStorage to make most of
the methods synchronous. WebappRegistry is now a singleton instance that
is instantiated at browser startup. This allows all SharedPreferences files to
be pre-warmed before the class is used; new web apps open new
SharedPreferences on a background thread when registered, after which the
preferences are cached automatically.

Most static methods on WebappRegistry and WebappDataStorage have been
converted to instance methods or removed. This makes the code much
cleaner and more efficient; each static method had to independently open
their SharedPreferences, which minimally performs a stat() on the
underlying XML file to see if it has changed. Now the singleton
WebappRegistry caches all WebappDataStorage objects on startup and
whenever new ones are added. This reduces disk IO overhead.

This CL allows all calls to SharedPreferences.Editor.apply() in
WebappRegistry and WebappDataStorage to occur on the main thread,
mostly removing the need for unwieldy callback interfaces and bare
pointer passing across the JNI.

BUG=633791

Review-Url: https://codereview.chromium.org/2351113005
Cr-Commit-Position: refs/heads/master@{#422703}