GIF: Fill the frame if we have a dummy color table

Since we will not draw anything later, we need to fill the frame with
the fill color.

NOTREECHECKS=true

BUG=skia:5883
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2450943002

Review-Url: https://codereview.chromium.org/2450943002
diff --git a/src/codec/SkGifCodec.cpp b/src/codec/SkGifCodec.cpp
index ec965cb..46a38cd 100644
--- a/src/codec/SkGifCodec.cpp
+++ b/src/codec/SkGifCodec.cpp
@@ -300,9 +300,11 @@
             // - The frame is interlaced. There is no obvious way to fill
             //   afterwards for an incomplete image. (FIXME: Does the first pass
             //   cover all rows? If so, we do not have to fill here.)
+            // - There is no color table for this frame. In that case will not
+            //   draw anything, so we need to fill.
             if (frameContext->frameRect() != this->getInfo().bounds()
                     || frameContext->transparentPixel() < SK_MAX_COLORS
-                    || frameContext->interlaced()) {
+                    || frameContext->interlaced() || !fCurrColorTableIsReal) {
                 // fill ignores the width (replaces it with the actual, scaled width).
                 // But we need to scale in Y.
                 const int scaledHeight = get_scaled_dimension(dstInfo.height(),