add SkColorSpace param to SkPixmap::erase()

We don't have a way to erase with a non-sRGB color.

Update the P3 gm to test this, removing the SkBitmap erase case where
there's no option for even an SkColor4f, let alone non-sRGB.  I'm not
sure it's really important to have one when we've got this on pixmap.

Updated release notes.

Change-Id: Ie98270d3f83e041593b4c6b2da8e325b36d4ff18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285341
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index fc3f1b9..921883e 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -8,6 +8,12 @@
 
   * <insert new release note here>
 
+  * SkBitmap and SkPixmap's erase() methods now treat their color parameters
+    consistently with the rest of Skia, with all SkColors and any untagged
+    SkColor4fs interpreted as sRGB, not as a color in the bitmap's color space.
+    SkPixmap::erase(SkColor4f) now takes an SkColorSpace, so you can pass
+    pixmap.colorSpace() if you want the old behavior.
+
   * SkCamera.h and SkMatrix44.h are DEPRECATED.
     Use SkM44 if you want to have 3d transformations.