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

include/core/SkTypes.h

Index: include/core/SkTypes.h
diff --git a/include/core/SkTypes.h b/include/core/SkTypes.h
index d7a791163be3617b4c414d3f30d4da0982b141d0..a94a59233f6b6596898aa90b1938e7126a092e0d 100644
--- a/include/core/SkTypes.h
+++ b/include/core/SkTypes.h
@@ -342,13 +342,15 @@ typedef uint32_t SkFourByteTag;
 /** 32 bit integer to hold a unicode value
 */
 typedef int32_t SkUnichar;
-/** 32 bit value to hold a millisecond count
-*/
+
+/** 32 bit value to hold a millisecond duration
+ *  Note that SK_MSecMax is about 25 days.
+ */
 typedef uint32_t SkMSec;
 /** 1 second measured in milliseconds
 */
 #define SK_MSec1 1000
-/** maximum representable milliseconds
+/** maximum representable milliseconds; 24d 20h 31m 23.647s.
 */
 #define SK_MSecMax 0x7FFFFFFF
 /** Returns a < b for milliseconds, correctly handling wrap-around from 0xFFFFFFFF to 0