Remove default for --images in common flags

Remove default for --images in common flags

Committed: https://skia.googlesource.com/skia/+/73bb435a5896d47f633f5e58e299b88d086c71c0

[bsalomon, 2015-06-30 15:39:57]

bsalomon@google.com changed reviewers:
+ mtklein@google.com, scroggo@google.com

[scroggo, 2015-06-30 15:43:51]

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

[scroggo, 2015-06-30 15:43:51]

Matt, it looks like you made resources/ the default image directory for
nanobench (in addition to DM, where it was already the default) in
https://codereview.chromium.org/918673002 (DIFFERENT ISSUE). Is it okay with you
if we remove the default? Do the bots already specify a value for --images?

[msarett, 2015-06-30 16:25:30]

lgtm

Yes!  Looks good.  It's my understanding that the bots use the gsutil images, so
I don't think there is a problem with not specifying a default.

[bsalomon, 2015-06-30 16:27:39]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2015-06-30 16:27:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1214363002/1

[commit-bot: I haz the power, 2015-06-30 16:38:40]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/73bb435a5896d47f633f5e58e299b88d086c71c0