DM: duh, don't calculate digests unless we're going to look at them.

dm/DMComparisonTask.cpp

 #include "DMComparisonTask.h"
 #include "DMUtil.h"
 
 namespace DM {
 
 ComparisonTask::ComparisonTask(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 ComparisonTask::draw() {
-    const skiagm::GmResultDigest digest(fBitmap);
-    if (!meetsExpectations(fExpectations, digest)) {
+    if (!meetsExpectations(fExpectations, fBitmap)) {
         this->fail();
     }
 }
 
 }  // namespace DM