Disable testcase7.bmp on SkImageDecoder and clean up dm_flags

Disable testcase7.bmp on SkImageDecoder and clean up dm_flags

testcase7.bmp leaves uninitialized memory.

Also remove "subset" dm flags.  The "subset" test no longer
exists.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/44509faecef54ba57e56d60dfc0b91cfefba2d5c

[msarett, 2015-11-16 18:01:49]

msarett@google.com changed reviewers:
+ scroggo@google.com

[scroggo, 2015-11-16 20:34:53]

1gtm, but first you need to update dm_flags.json (I think by running
dm_flags.py?)

[msarett, 2015-11-16 20:39:36]

Thanks, adding the changes to the json.

[scroggo, 2015-11-17 15:23:45]

lgtm

[msarett, 2015-11-17 15:24:19]

The CQ bit was checked by msarett@google.com

[commit-bot: I haz the power, 2015-11-17 15:24:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1446303002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1446303002/20001

[commit-bot: I haz the power, 2015-11-17 15:36:13]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/44509faecef54ba57e56d60dfc0b91cfefba2d5c