Rename parameter `isAlphaMask` to `is_mask` in SetBitmapPaint()

https://pdfium-review.googlesource.com/75435 previously renamed function
CFX_DIBBase::IsAlphaMask() to IsMask() since it only checks whether the
bitmap is in mask format.

This CL renames SetBitmapPaint()'s input parameter from `isAlphaMask` to
`is_mask` since its callers use CFX_DIBBase::IsMask() as the inputs.

Change-Id: I37bd5b73a57544090a8c14fa3f471c1e060fcfdb
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/92030
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Nigi <nigi@chromium.org>
diff --git a/core/fxge/skia/fx_skia_device.cpp b/core/fxge/skia/fx_skia_device.cpp
index cc5bbf1..b8545d2 100644
--- a/core/fxge/skia/fx_skia_device.cpp
+++ b/core/fxge/skia/fx_skia_device.cpp
@@ -625,14 +625,14 @@
                    -m.d / height, m.d + m.f, 0, 0, 1);
 }
 
-void SetBitmapPaint(bool isAlphaMask,
+void SetBitmapPaint(bool is_mask,
                     bool anti_alias,
                     uint32_t argb,
                     int bitmap_alpha,
                     BlendMode blend_type,
                     SkPaint* paint) {
   paint->setAntiAlias(anti_alias);
-  if (isAlphaMask)
+  if (is_mask)
     paint->setColorFilter(SkColorFilters::Blend(argb, SkBlendMode::kSrc));
 
   // paint->setFilterQuality(kHigh_SkFilterQuality);