Implement Fill() for incomplete decodes to RGBA_F16

Before this patch, we would hit an SkASSERT(false).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2335203002

Review-Url: https://codereview.chromium.org/2335203002
diff --git a/src/codec/SkBmpStandardCodec.h b/src/codec/SkBmpStandardCodec.h
index 506742b..7039cf7 100644
--- a/src/codec/SkBmpStandardCodec.h
+++ b/src/codec/SkBmpStandardCodec.h
@@ -57,7 +57,7 @@
             int* inputColorCount) override;
 
 
-    uint32_t onGetFillValue(SkColorType) const override;
+    uint64_t onGetFillValue(const SkImageInfo&) const override;
 
     SkSampler* getSampler(bool createIfNecessary) override {
         SkASSERT(fSwizzler);