Fix a warning building DM using ninja on Mac.

Here is the warning:
../../dm/DMTask.cpp: In copy constructor ‘DM::Task::Task(const DM::Task&)’:
../../dm/DMTask.cpp:17: warning: base class ‘class SkRunnable’ should be explicitly initialized in the copy constructor

Also add an SK_OVERRIDE.

R=mtklein@google.com

Review URL: https://codereview.chromium.org/76903002

git-svn-id: http://skia.googlecode.com/svn/trunk@12317 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/dm/DMTask.cpp b/dm/DMTask.cpp
index 9b463f9..ba74a5f 100644
--- a/dm/DMTask.cpp
+++ b/dm/DMTask.cpp
@@ -12,7 +12,10 @@
     fReporter->start();
 }
 
-Task::Task(const Task& that) : fReporter(that.fReporter), fTaskRunner(that.fTaskRunner) {
+Task::Task(const Task& that)
+    : INHERITED(that)
+    , fReporter(that.fReporter)
+    , fTaskRunner(that.fTaskRunner) {
     fReporter->start();
 }