Add software backend to gpu build

Bug: skia:8548
Change-Id: If3853711f4b183f3b0437ae2b31525b0e6c88213
Reviewed-on: https://skia-review.googlesource.com/c/172120
Reviewed-by: Kevin Lubick <kjlubick@google.com>
diff --git a/experimental/canvaskit/compile.sh b/experimental/canvaskit/compile.sh
index 5979f98..881f7f4 100755
--- a/experimental/canvaskit/compile.sh
+++ b/experimental/canvaskit/compile.sh
@@ -36,7 +36,7 @@
 GN_GPU="skia_enable_gpu=true"
 GN_GPU_FLAGS="\"-DIS_WEBGL=1\", \"-DSK_DISABLE_LEGACY_SHADERCONTEXT\","
 WASM_GPU="-lEGL -lGLESv2 -DSK_SUPPORT_GPU=1 \
-          -DSK_DISABLE_LEGACY_SHADERCONTEXT --pre-js $BASE_DIR/gpu.js"
+          -DSK_DISABLE_LEGACY_SHADERCONTEXT --pre-js $BASE_DIR/cpu.js --pre-js $BASE_DIR/gpu.js"
 if [[ $@ == *cpu* ]]; then
   echo "Using the CPU backend instead of the GPU backend"
   GN_GPU="skia_enable_gpu=false"