Fix filesystem API file_handlers to work for drive on ChromeOS.

chrome/browser/chromeos/drive/drive_file_system_proxy.cc

Index: chrome/browser/chromeos/drive/drive_file_system_proxy.cc
diff --git a/chrome/browser/chromeos/drive/drive_file_system_proxy.cc b/chrome/browser/chromeos/drive/drive_file_system_proxy.cc
index 6a616508d9bc2d70940b190af5ccd2646b05cbe9..100a4aafbece635d9882b47b1d37e748a7f5d16d 100644
--- a/chrome/browser/chromeos/drive/drive_file_system_proxy.cc
+++ b/chrome/browser/chromeos/drive/drive_file_system_proxy.cc
@@ -697,8 +697,15 @@ bool DriveFileSystemProxy::ValidateUrl(
   if (!url.is_valid() || url.type() != fileapi::kFileSystemTypeDrive) {
     return false;
   }
-  *file_path = url.virtual_path();
-  return true;
+
+  // |url.virtual_path()| cannot be used directly because in the case the url is
+  // isolated file system url, the virtual path will be formatted as
+  // <isolated_file_system_id>/<file_name> and thus unusable by drive file
+  // system.
+  // TODO(kinaba): fix other uses of virtual_path() as in
+  // https://codereview.chromium.org/12483010/
+  *file_path = util::ExtractDrivePath(url.path());
+  return !file_path->empty();
 }
 
 void DriveFileSystemProxy::CallDriveFileSystemMethodOnUIThread(