[SurfaceFlinger] Replace NULL with nullptr.

Test: make
Change-Id: Ia0ecb3160cd5fddf22fb7100c1759d62e723d0b7
diff --git a/services/surfaceflinger/FrameTracker.h b/services/surfaceflinger/FrameTracker.h
index adcdfb5..b4a9fd6 100644
--- a/services/surfaceflinger/FrameTracker.h
+++ b/services/surfaceflinger/FrameTracker.h
@@ -35,7 +35,7 @@
 // possible.
 //
 // Some of the time values tracked may be set either as a specific timestamp
-// or a fence.  When a non-NULL fence is set for a given time value, the
+// or a fence.  When a non-nullptr fence is set for a given time value, the
 // signal time of that fence is used instead of the timestamp.
 class FrameTracker {