Remove SkSize& SkSize::operator=(const SkISize&)

Having it means you cannot write:

SkSize size;
float x, y;
size = {x , y};

clang allows it but GCC does not, claiming it is ambiguous between the
implicitly generated
SkSize& SkSize::operator=(const SkSize&)
  and
SkSize& SkSize::operator=(const SkISize&)

clang gives the same error if the former is explicitly declared default.

Change-Id: I3b64436ef6aa669b3d87e7f37057c5dcb4add987
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257880
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index 4262438..a1d443e 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -7,6 +7,9 @@
 Milestone 80
 
 <Insert new notes here- top is most recent.>
+  * Removed SkSize& SkSize::operator=(const SkISize&)
+    https://review.skia.org/257880
+
   * SkISize width() and height() now constexpr
     https://review.skia.org/257680