Remove error handling from MipmapUtil and add DCHECKs

cc/tiles/software_image_decode_controller.cc

Index: cc/tiles/software_image_decode_controller.cc
diff --git a/cc/tiles/software_image_decode_controller.cc b/cc/tiles/software_image_decode_controller.cc
index a44d7d0feecc309d5ebe748df60c1a1876a37815..4d0ca7e851711a6583bb48517264e029ec11555c 100644
--- a/cc/tiles/software_image_decode_controller.cc
+++ b/cc/tiles/software_image_decode_controller.cc
@@ -818,7 +818,6 @@ ImageDecodeControllerKey ImageDecodeControllerKey::FromDrawImage(
   if (quality == kMedium_SkFilterQuality && !target_size.IsEmpty()) {
     SkSize mip_target_size =
         MipMapUtil::GetScaleAdjustmentForSize(src_rect.size(), target_size);
-    DCHECK(mip_target_size.width() != -1.f && mip_target_size.height() != -1.f);
     target_size.set_width(src_rect.width() * mip_target_size.width());
     target_size.set_height(src_rect.height() * mip_target_size.height());
   }