Fix -Wdeprecated-copy warning with CFX_RectF.

This appears with the latest Clang used by Chromium, but not in the
slightly older Clang version used by PDFium.

Change-Id: I314418db34ed201d01a1a7c4327fcca6f17dd45a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/82010
Auto-Submit: Lei Zhang <thestig@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>

core/fxcrt/fx_coordinates.h

diff --git a/core/fxcrt/fx_coordinates.h b/core/fxcrt/fx_coordinates.h
index e458380..4a7d33c 100644
--- a/core/fxcrt/fx_coordinates.h
+++ b/core/fxcrt/fx_coordinates.h
@@ -339,6 +339,8 @@
   // NOLINTNEXTLINE(runtime/explicit)
   CFX_RectF(const CFX_RectF& other) = default;
 
+  CFX_RectF& operator=(const CFX_RectF& other) = default;
+
   CFX_RectF& operator+=(const PointType& p) {
     left += p.x;
     top += p.y;