Experiment to track coverage in a layer

Bug: skia:
Change-Id: I5ed334f63e64991944394dc8103092a2c6280546
Reviewed-on: https://skia-review.googlesource.com/122000
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/docs/SkCanvas_Reference.bmh b/docs/SkCanvas_Reference.bmh
index de33770..0bce15e 100644
--- a/docs/SkCanvas_Reference.bmh
+++ b/docs/SkCanvas_Reference.bmh
@@ -1684,6 +1684,7 @@
     enum {
         kPreserveLCDText_SaveLayerFlag = 1 << 1,
         kInitWithPrevious_SaveLayerFlag = 1 << 2,
+        kMaskAgainstCoverage_EXPERIMENTAL_DONT_USE_SaveLayerFlag = 1 << 3,
         kDontClipToLayer_Legacy_SaveLayerFlag = kDontClipToLayer_PrivateSaveLayerFlag,
     };
 ##
@@ -1700,6 +1701,10 @@
   Initializes Layer with the contents of the previous Layer.
 ##
 
+#Const kMaskAgainstCoverage_EXPERIMENTAL_DONT_USE_SaveLayerFlag 8
+  Experimental -- don't use
+##
+
 #Const kDontClipToLayer_Legacy_SaveLayerFlag 0x80000000
 #Deprecated soon
 ##