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
2 files changed