additional serialize(void*, size_t) to match SkTextBlob

Add serialize(void*, size_t) to mirror the SkTextBlob serialization
methods.

Bug: chromium:1278340

Change-Id: Ie3868d882263a0ec8833d4c9404e7410d30ac6ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/511798
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>

include/private/chromium/GrSlug.h

@@ -47,6 +47,7 @@ public:
 
     // Serialize the slug.
     sk_sp<SkData> serialize() const;
+    size_t serialize(void* buffer, size_t size) const;
 
     // Set the client parameter to the appropriate SkStrikeClient when typeface ID translation
     // is needed.

src/gpu/text/GrSlug.cpp

@@ -25,6 +25,17 @@ sk_sp<SkData> GrSlug::serialize() const {
     return buffer.snapshotAsData();
 }
 
+size_t GrSlug::serialize(void* buffer, size_t size) const {
+    SkBinaryWriteBuffer writeBuffer{buffer, size};
+    this->doFlatten(writeBuffer);
+
+    // If we overflow the given buffer, then SkWriteBuffer allocates a new larger buffer. Check
+    // to see if an additional buffer was allocated, if it wasn't then everything fit, else
+    // return 0 signaling the buffer overflowed.
+    // N.B. This is the idiom from SkTextBlob.
+    return writeBuffer.usingInitialStorage() ? writeBuffer.bytesWritten() : 0u;
+}
+
 sk_sp<GrSlug> SkMakeSlugFromBuffer(SkReadBuffer& buffer, const SkStrikeClient* client);
 sk_sp<GrSlug> GrSlug::Deserialize(const void* data, size_t size, const SkStrikeClient* client) {
     SkReadBuffer buffer{data, size};