skip unneeded header from writeFlattenable when building font-cache-key. Saves ~30% in size

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1924033002

TBR=mtklein

Review-Url: https://codereview.chromium.org/1924033002
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index 123f203..d28168a 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -1567,14 +1567,14 @@
     size_t descSize = sizeof(*rec);
 
     if (pe) {
-        peBuffer->writeFlattenable(pe);
+        pe->flatten(*peBuffer);
         descSize += peBuffer->bytesWritten();
         entryCount += 1;
         rec->fMaskFormat = SkMask::kA8_Format;  // force antialiasing when we do the scan conversion
         // seems like we could support kLCD as well at this point...
     }
     if (mf) {
-        mfBuffer->writeFlattenable(mf);
+        mf->flatten(*mfBuffer);
         descSize += mfBuffer->bytesWritten();
         entryCount += 1;
         rec->fMaskFormat = SkMask::kA8_Format;   // force antialiasing with maskfilters
@@ -1585,7 +1585,7 @@
         rec->ignorePreBlend();
     }
     if (ra) {
-        raBuffer->writeFlattenable(ra);
+        ra->flatten(*raBuffer);
         descSize += raBuffer->bytesWritten();
         entryCount += 1;
         rec->fMaskFormat = SkMask::kA8_Format;  // force antialiasing when we do the scan conversion