Remove an ASSERT() in CFX_DIBitmap::ConvertColorScale().

- It can fail.
- The next line handles the failure anyway.

BUG=chromium:930489

Change-Id: Icec60fef85461ef15994bc7001a06b6ad2dcb800
Reviewed-on: https://pdfium-review.googlesource.com/c/50570
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: 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 561009e..b27a75f 100644
--- a/core/fxge/dib/cfx_dibitmap.cpp
+++ b/core/fxge/dib/cfx_dibitmap.cpp
@@ -821,7 +821,6 @@
 }
 
 bool CFX_DIBitmap::ConvertColorScale(uint32_t forecolor, uint32_t backcolor) {
-  ASSERT(!IsAlphaMask());
   if (!m_pBuffer || IsAlphaMask())
     return false;