media/gpu/VDA unittest: Fix wrong USE_VAAPI macro

This wrong macro causes GenericDmaBufVideoFrameValidator is used on intel platform.
Due to it, VDA unittest with frame validator fails on intel platform.

BUG=chromium:856562
TEST=VDA unittest --frame_validator=check on eve

Change-Id: Iad1749ea4ada09a9d01b92137ca10c3215d9893d
Reviewed-on: https://chromium-review.googlesource.com/c/1343404
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609577}

media/gpu/video_decode_accelerator_unittest.cc

@@ -1160,7 +1160,7 @@ CreateAndInitializeVideoFrameValidator(
       prefix_output_yuv = GetTestDataFile(filepath);
     }
   }
-#if defined(USE_VAAPI)
+#if BUILDFLAG(USE_VAAPI)
   bool linear = false;
 #else
   bool linear = true;