Change SkTime::GetMSecs to double; ensure values stored in SkMSec do not overflow.

tests/skia_test.cpp

Index: tests/skia_test.cpp
diff --git a/tests/skia_test.cpp b/tests/skia_test.cpp
index 65fbc3284297a9070737fbacd3994e08ac2502f5..5d21d1ec15fba425e0a2ae39defb7aa4bf6b9d56 100644
--- a/tests/skia_test.cpp
+++ b/tests/skia_test.cpp
@@ -95,9 +95,9 @@ public:
           int fTestCount;
       } reporter;
 
-      const SkMSec start = SkTime::GetMSecs();
+      const Timer timer;
       fTest.proc(&reporter, fGrContextFactory);
-      SkMSec elapsed = SkTime::GetMSecs() - start;
+      SkMSec elapsed = timer.elapsedMsInt();
       if (reporter.fError) {
           fStatus->reportFailure();
       }