Implement onSkipScanlines() for bmp and wbmp

TBR=reed@google.com
BUG=skia:4270
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1691083002

Review URL: https://codereview.chromium.org/1691083002
diff --git a/src/codec/SkWbmpCodec.cpp b/src/codec/SkWbmpCodec.cpp
index 4dd0f25..af1ab46 100644
--- a/src/codec/SkWbmpCodec.cpp
+++ b/src/codec/SkWbmpCodec.cpp
@@ -181,6 +181,11 @@
     return count;
 }
 
+bool SkWbmpCodec::onSkipScanlines(int count) {
+    const size_t bytesToSkip = count * fSrcRowBytes;
+    return this->stream()->skip(bytesToSkip) == bytesToSkip;
+}
+
 SkCodec::Result SkWbmpCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
         const Options& options, SkPMColor inputColorTable[], int* inputColorCount) {
     if (options.fSubset) {