Add support for reading a directory of images with --expectations (-r).

DM writes out its images in a hierarchy that's a little different than GM,
so this can't read GM's output.  But it can read its own, written with -w.

Example usage:
  $ out/Release/dm -w /tmp/baseline
  $ out/Release/dm -r /tmp/baseline -w /tmp/new
  (and optionally)
  $ mkdir /tmp/diff; out/Release/skdiff /tmp/baseline /tmp/new /tmp/diff

GM's IndividualImageExpectationsSource and Expectations are a little too eager
about decoding and hashing the expected images, so I took the opportunity to
add DM::Expectations that mostly replaces skiagm::ExpectationsSource and
skiagm::Expectations in DM.  It mainly exists to move the image decoding and
comparison off the main thread, which would otherwise be a major speed
bottleneck.

I tried to use skiagm code where possible.  One notable place where I differed
is in this new feature.  When -r is a directory of images, DM does no hashing.
It considerably faster to read the expected file into an SkBitmap and do a
byte-for-byte comparison than to hash the two bitmaps and check those.

The example usage above isn't quite working 100% yet.  Expectations on some GMs
fail, even with no binary change.  I haven't pinned down whether this is due to
  - a bug in DM
  - flaky GMs
  - unthreadsafe GMs
  - flaky image decoding
  - unthreadsafe image decoding
  - something else
but I intend to.  Leon, Derek and I have suspected PNG decoding isn't
threadsafe, but are as yet unable to prove it.

I also seem to be able to cause malloc to fail on my laptop if I run too many
configs at once, though I never seem to be using more than ~1G of RAM.  Will
track that down too.

BUG=
R=reed@google.com, bsalomon@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/108963002

git-svn-id: http://skia.googlecode.com/svn/trunk@12596 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/dm/DMCpuTask.cpp b/dm/DMCpuTask.cpp
index 3f51c8a..c538f0a 100644
--- a/dm/DMCpuTask.cpp
+++ b/dm/DMCpuTask.cpp
@@ -1,5 +1,5 @@
 #include "DMCpuTask.h"
-#include "DMChecksumTask.h"
+#include "DMExpectationsTask.h"
 #include "DMPipeTask.h"
 #include "DMReplayTask.h"
 #include "DMSerializeTask.h"
@@ -12,14 +12,14 @@
 CpuTask::CpuTask(const char* name,
                  Reporter* reporter,
                  TaskRunner* taskRunner,
-                 const skiagm::ExpectationsSource& expectations,
+                 const Expectations& expectations,
                  skiagm::GMRegistry::Factory gmFactory,
                  SkBitmap::Config config)
     : Task(reporter, taskRunner)
     , fGMFactory(gmFactory)
     , fGM(fGMFactory(NULL))
     , fName(UnderJoin(fGM->shortName(), name))
-    , fExpectations(expectations.get(Png(fName).c_str()))
+    , fExpectations(expectations)
     , fConfig(config)
     {}
 
@@ -33,7 +33,7 @@
     canvas.flush();
 
 #define SPAWN(ChildTask, ...) this->spawnChild(SkNEW_ARGS(ChildTask, (*this, __VA_ARGS__)))
-    SPAWN(ChecksumTask, fExpectations, bitmap);
+    SPAWN(ExpectationsTask, fExpectations, bitmap);
 
     SPAWN(PipeTask, fGMFactory(NULL), bitmap, false, false);
     SPAWN(PipeTask, fGMFactory(NULL), bitmap, true, false);