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The previous check location was forcing all apps and extensions which call
the chrome.fileSystem.requestFileSystem API, to have "fileBrowserHandler"
permission as well.

That however doesn't make much sense, as those apps are not always handlers.
This CL moves the check to FileBrowserHandler::GetHandler() so simply the
permission has to be used only if the app wants to be a handler.

TEST=unit_test: FileBrowserHandlerManifestTest.GetHandlersRequiresPermission
BUG=470494

Review URL: https://codereview.chromium.org/1030133002

Cr-Commit-Position: refs/heads/master@{#322343}