Do some minor pre cleanup work before converting all xfermodes to XPs.

src/effects/SkXfermodeImageFilter.cpp

Index: src/effects/SkXfermodeImageFilter.cpp
diff --git a/src/effects/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index 617593eae6e559b300a6f435f6f0bb0642b4a74f..b31cd80efd9f0a832104b4ae84e7e9ab2104283c 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -112,6 +112,9 @@ void SkXfermodeImageFilter::toString(SkString* str) const {
 #if SK_SUPPORT_GPU
 
 bool SkXfermodeImageFilter::canFilterImageGPU() const {
+    // This is checked with the background texture as NULL here since we have no actual texture,

[bsalomon, 2015/01/14 15:18:13]

Does this comment add much? Maybe just "Call asFragmentProcessor() with NULLs to
use it as a query without creating a processor."

+    // however it will be assumed that a valid texture is used if asFragmentProcessor is called
+    // outside of checking return values.
     return fMode && fMode->asFragmentProcessor(NULL, NULL) && !cropRectIsSet();
 }
 
@@ -127,6 +130,12 @@ bool SkXfermodeImageFilter::filterImageGPU(Proxy* proxy,
         return onFilterImage(proxy, src, ctx, result, offset);
     }
     GrTexture* backgroundTex = background.getTexture();
+
+    if (NULL == backgroundTex) {
+        SkASSERT(false);
+        return false;
+    }
+
     SkBitmap foreground = src;
     SkIPoint foregroundOffset = SkIPoint::Make(0, 0);
     if (getInput(1) && !getInput(1)->getInputResultGPU(proxy, src, ctx, &foreground,