Files.app: Fix DCHECK in FakeFileSystem.

Previous DCHECK uses IsUnderDriveMountPoint method which takes paths for
external file system. But the paths used in FakeFileSystem are drive paths that
are extracted from external file system paths. So it goes into infinity loop
during traversing parent directory when a path pointing the out of my drive
directory is given.

The CL removes the check, and add util::GetDriveMyDriveRootPath().IsParent(file_path) to
prevent the infinity loop.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/583583003

Cr-Commit-Position: refs/heads/master@{#296644}

chrome/browser/chromeos/drive/fake_file_system.cc

@@ -146,9 +146,6 @@ void FakeFileSystem::GetResourceEntry(
     const GetResourceEntryCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
-  // Now, we only support files under my drive.
-  DCHECK(!util::IsUnderDriveMountPoint(file_path));
-
   if (file_path == util::GetDriveMyDriveRootPath()) {
     // Specialized for the root entry.
     drive_service_->GetAboutResource(
@@ -158,6 +155,8 @@ void FakeFileSystem::GetResourceEntry(
     return;
   }
 
+  // Now, we only support files under my drive.
+  DCHECK(util::GetDriveMyDriveRootPath().IsParent(file_path));
   GetResourceEntry(
       file_path.DirName(),
       base::Bind(