Test CodecImageGenerator on GPU bots

In crrev.com/1549473003, CodecImageGenerator implemented getYUV8Planes,
so that we can test on a GPU bot. Update the arguments to DM so that
we run CodecImageGenerator on GPU bots.

This is adapted from:
https://codereview.chromium.org/1663453002/

This will cause many untriaged images on Gold.
The GPU converts YUV->RGBA differently than we
do in software.

BUG=skia:4888
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1676663002

Review URL: https://codereview.chromium.org/1676663002
diff --git a/tools/dm_flags.py b/tools/dm_flags.py
index 5fd8a2b..663507e 100755
--- a/tools/dm_flags.py
+++ b/tools/dm_flags.py
@@ -80,13 +80,9 @@
   args.append('--config')
   args.extend(configs)
 
-  # Run tests and gms everywhere,
-  # and image decoding tests everywhere except GPU bots.
+  # Run tests, gms, and image decoding tests everywhere.
   # TODO: remove skp from default --src list?
-  if 'GPU' in bot:
-    args.extend('--src tests gm'.split(' '))
-  else:
-    args.extend('--src tests gm image'.split(' '))
+  args.extend('--src tests gm image'.split(' '))
 
   if 'GalaxyS' in bot:
     args.extend(('--threads', '0'))