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This CL reorganizes code before adding the read directory implementation.

First of all, operations are now extracted to separate classes, to avoid huge
file_system_provider.cc. Such operation classes are now arguments for the
RequestManager which replaces the former pair of [success, error] callbacks.

Such operation classes are now easy to test, because of ability to cut out
the EventRouter by using Operation::SetDispatchEventImplForTests.

Finally, a RequestValue class has been introduced, which replaces former
base::DictionaryValue. The former solution was not good, since we already
parse the base::Value in api function implementations (using generated from
IDL parsers). So, passing a raw base::Value and later parsing is again is an
overkill.

The RequestValue class is a trivial wrapper for all kinds of IDL values, which
can be passed through the RequestManager to Operation classes.

TEST=unit_tests, browser_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/257493004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266516 0039d316-1c4b-4281-b951-d872f2087c98