Fix an invalid DCHECK in SharedImageVideo.

A DCHECK became invalid after a recent refactoring. Move the DCHECK to
more appropriate place.
BeginReadAccess() could be called multiple times on a single shared
image in which case this DCHECK is valid only for the first call. once
|promise_texture_| is created and cached, this DCHECK becomes invalid.

Change-Id: Ia77759ca7797675adbcf41a78c1086e3aebba2a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822318Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: vikas soni <vikassoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699903}

gpu/command_buffer/service/shared_image_video.cc

@@ -329,8 +329,8 @@ class SharedImageRepresentationVideoSkiaVk
         LOG(ERROR) << "Failed to get the hardware buffer.";
         return nullptr;
       }
-    }
       DCHECK(scoped_hardware_buffer_->buffer());
+    }
 
     // Wait on the sync fd attached to the buffer to make sure buffer is
     // ready before the read. This is done by inserting the sync fd semaphore