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

dm/DMChecksumTask.cpp

-#include "DMChecksumTask.h"
-#include "DMUtil.h"
-
-namespace DM {
-
-ChecksumTask::ChecksumTask(const Task& parent,
-                           skiagm::Expectations expectations,
-                           SkBitmap bitmap)
-    : Task(parent)
-    , fName(parent.name())  // Masquerade as parent so failures are attributed to it.
-    , fExpectations(expectations)
-    , fBitmap(bitmap)
-    {}
-
-void ChecksumTask::draw() {
-    if (fExpectations.ignoreFailure() || fExpectations.empty()) {
-        return;
-    }
-
-    const skiagm::GmResultDigest digest(fBitmap);
-    if (!fExpectations.match(digest)) {
-        this->fail();
-    }
-}
-
-}  // namespace DM