Extract external file systems handling from isolated context.

webkit/fileapi/file_system_mount_point_provider.h

Index: webkit/fileapi/file_system_mount_point_provider.h
diff --git a/webkit/fileapi/file_system_mount_point_provider.h b/webkit/fileapi/file_system_mount_point_provider.h
index 4f0950e29ce5ad9acd7613c40cdab0e865aa81fe..24c00fe0e6a930df1d689b4c6ee84554ca700ff3 100644
--- a/webkit/fileapi/file_system_mount_point_provider.h
+++ b/webkit/fileapi/file_system_mount_point_provider.h
@@ -141,14 +141,14 @@ class ExternalFileSystemMountPointProvider
   virtual void RevokeAccessForExtension(
         const std::string& extension_id) = 0;
   // Checks if a given |mount_point| already exists.
-  virtual bool HasMountPoint(const FilePath& mount_point) = 0;
+  virtual bool HasMountPoint(const FilePath& mount_point) const = 0;
   // Adds a new local mount point.
-  virtual void AddLocalMountPoint(const FilePath& mount_point) = 0;
+  virtual bool AddLocalMountPoint(const FilePath& mount_point) = 0;
   // Adds a new local mount point that will be accessible only by extensions
   // that have been granted full acess for all external file systems.
-  virtual void AddRestrictedLocalMountPoint(const FilePath& mount_point) = 0;
+  virtual bool AddRestrictedLocalMountPoint(const FilePath& mount_point) = 0;
   // Adds a new remote mount point.
-  virtual void AddRemoteMountPoint(
+  virtual bool AddRemoteMountPoint(
       const FilePath& mount_point,
       RemoteFileSystemProxyInterface* remote_proxy) = 0;
   // Removes a mount point.