Use IsOpaqueImage() in CFX_DIBitmap::MultiplyAlpha().

There already is a method to cover this exact expression, so use it.

Change-Id: Ie285179ccf15450482cba9b9ea2d4be3addec0c1
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/75436
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fxge/dib/cfx_dibitmap.cpp

diff --git a/core/fxge/dib/cfx_dibitmap.cpp b/core/fxge/dib/cfx_dibitmap.cpp
index d8da3ba..cb95cae 100644
--- a/core/fxge/dib/cfx_dibitmap.cpp
+++ b/core/fxge/dib/cfx_dibitmap.cpp
@@ -438,7 +438,7 @@
     return false;
   }
 
-  if (!IsMask() && !HasAlpha())
+  if (IsOpaqueImage())
     return LoadChannelFromAlpha(FXDIB_Alpha, pSrcBitmap);
 
   RetainPtr<CFX_DIBitmap> pSrcClone = pSrcBitmap.As<CFX_DIBitmap>();