GM: add --writeChecksumBasedFilenames option

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

Index: gm/tests/outputs/checksum-based-filenames/output-expected/stdout
===================================================================
--- gm/tests/outputs/checksum-based-filenames/output-expected/stdout	(revision 0)
+++ gm/tests/outputs/checksum-based-filenames/output-expected/stdout	(revision 0)
@@ -0,0 +1,17 @@
+GM: These configs will be run: 8888 565
+GM: reading expectations from JSON summary file gm/tests/inputs/json/different-pixels-no-hierarchy.json
+GM: writing to gm/tests/outputs/checksum-based-filenames/output-actual/writePath
+GM: writing mismatches to gm/tests/outputs/checksum-based-filenames/output-actual/mismatchPath
+GM: drawing... selftest1 [300 200]
+GM: Ran 1 GMs
+GM: ... over  2 configs ["8888", "565"]
+GM: ...  and  7 modes   ["pipe", "pipe cross-process", "pipe cross-process, shared address", "replay", "rtree", "serialize", "tilegrid"]
+GM: ... so there should be a total of 9 tests.
+GM: Ran 9 tests: NoGpuContext=0 IntentionallySkipped=0 RenderModeMismatch=0 ExpectationsMismatch=2 MissingExpectations=0 WritingReferenceImage=0
+GM: [*] 0 NoGpuContext:
+GM: [ ] 0 IntentionallySkipped:
+GM: [*] 0 RenderModeMismatch:
+GM: [*] 2 ExpectationsMismatch: selftest1_8888 selftest1_565
+GM: [ ] 0 MissingExpectations:
+GM: [*] 0 WritingReferenceImage:
+GM: (results marked with [*] will cause nonzero return value)