Add tracing to saveLayer

I think this would be helpful for tracing as saveLayer is usually very costly.

Bug: skia:
Change-Id: Ia5bc194d6a7346c6fbec54da86065dc95d36f612
Reviewed-on: https://skia-review.googlesource.com/150115
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Auto-Submit: Yuqian Li <liyuqian@google.com>
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 19153ab..9566a34 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -920,6 +920,7 @@
 }
 
 int SkCanvas::saveLayer(const SaveLayerRec& rec) {
+    TRACE_EVENT0("skia", TRACE_FUNC);
     if (rec.fPaint && rec.fPaint->nothingToDraw()) {
         // no need for the layer (or any of the draws until the matching restore()
         this->save();
@@ -974,6 +975,7 @@
 }
 
 void SkCanvas::internalSaveLayer(const SaveLayerRec& rec, SaveLayerStrategy strategy) {
+    TRACE_EVENT0("skia", TRACE_FUNC);
     const SkRect* bounds = rec.fBounds;
     const SkPaint* paint = rec.fPaint;
     SaveLayerFlags saveLayerFlags = rec.fSaveLayerFlags;