GM: add --missingExpectationsPath option, similar to --mismatchPath

gm/tests/outputs/checksum-based-filenames/output-expected/command_line

Index: gm/tests/outputs/checksum-based-filenames/output-expected/command_line
===================================================================
--- gm/tests/outputs/checksum-based-filenames/output-expected/command_line	(revision 9724)
+++ gm/tests/outputs/checksum-based-filenames/output-expected/command_line	(working copy)
@@ -1 +1 @@
-out/Debug/gm --verbose --writeChecksumBasedFilenames --match selftest1 --config 8888 565 -r gm/tests/inputs/json/different-pixels-no-hierarchy.json --writeJsonSummaryPath gm/tests/outputs/checksum-based-filenames/output-actual/json-summary.txt --writePath gm/tests/outputs/checksum-based-filenames/output-actual/writePath --mismatchPath gm/tests/outputs/checksum-based-filenames/output-actual/mismatchPath
+out/Debug/gm --verbose --writeChecksumBasedFilenames --match selftest1 --config 8888 565 -r gm/tests/inputs/json/different-pixels-no-hierarchy.json --writeJsonSummaryPath gm/tests/outputs/checksum-based-filenames/output-actual/json-summary.txt --writePath gm/tests/outputs/checksum-based-filenames/output-actual/writePath --mismatchPath gm/tests/outputs/checksum-based-filenames/output-actual/mismatchPath --missingExpectationsPath gm/tests/outputs/checksum-based-filenames/output-actual/missingExpectationsPath

[epoger, 2013/06/21 17:46:44]

in all the command_line files, we see this new flag being passed

[borenet, 2013/06/21 18:01:14]

These expectations are getting really unwieldy.  Is there a good way to
simplify?

[epoger, 2013/06/21 18:06:46]

I agree, especially all the "bogusfile"s to make sure the directories aren't
empty.

1. Do you have a particular simplifying suggestion?

2. Do you think it would be reasonable to commit this change as-is and simplify
the test later, so we can start saving time on our buildbots?

[borenet, 2013/06/21 18:14:09]

1. Maybe modify the test so that it can handle empty or nonexistent directories?
 FWIW I think "whitespace.txt" is a better name for an empty placeholder file. 
It makes it clear that nobody needs to open it and read it.  For changes which
affect the buildbot factory config test, I typically won't include the
expectations files in the review, or I'll only include one, unless there are
some significant changes.

2. Yes, that's fine.