Reland "Deserialize MultiPictureDocument based SKP files (with image sharing proc) in wasm debugger."

This is a reland of 7635013ad16a60cfa5ff93af28c5fab8927c92ce

Original change's description:
> Deserialize MultiPictureDocument based SKP files (with image sharing proc) in wasm debugger.
> 
> Change-Id: I73affae3cd05a2aa6ac1c75c8e049d352bbf3a85
> Bug: 9176
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217135
> Commit-Queue: Nathaniel Nifong <nifong@google.com>
> Reviewed-by: Derek Sollenberger <djsollen@google.com>
> Reviewed-by: Kevin Lubick <kjlubick@google.com>

Bug: 9176
Change-Id: Ifef1ff45ac0013ba3015f88c7ecd75527b28b604
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222505
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
diff --git a/tools/SkSharingProc.cpp b/tools/SkSharingProc.cpp
new file mode 100644
index 0000000..803f766
--- /dev/null
+++ b/tools/SkSharingProc.cpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2019 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "tools/SkSharingProc.h"
+
+#include "include/core/SkData.h"
+#include "include/core/SkImage.h"
+#include "include/core/SkSerialProcs.h"
+
+sk_sp<SkData> SkSharingSerialContext::serializeImage(SkImage* img, void* ctx) {
+    SkSharingSerialContext* context = reinterpret_cast<SkSharingSerialContext*>(ctx);
+    uint32_t id = img->uniqueID(); // get this process's id for the image. these are not hashes.
+    // find out if we have already serialized this, and if so, what its in-file id is.
+    auto iter = context->fImageMap.find(id);
+    if (iter == context->fImageMap.end()) {
+        // When not present, add its id to the map and return its usual serialized form.
+        context->fImageMap[id] = context->fImageMap.size();
+        return img->encodeToData();
+    }
+    uint32_t fid = context->fImageMap[id];
+    // if present, return only the in-file id we registered the first time we serialized it.
+    return SkData::MakeWithCopy(&fid, sizeof(fid));
+}
+
+sk_sp<SkImage> SkSharingDeserialContext::deserializeImage(
+  const void* data, size_t length, void* ctx) {
+    SkSharingDeserialContext* context = reinterpret_cast<SkSharingDeserialContext*>(ctx);
+    uint32_t fid;
+    // If the data is an image fid, look up an already deserialized image from our map
+    if (length == sizeof(fid)) {
+        memcpy(&fid, data, sizeof(fid));
+        if (fid >= context->fImages.size()) {
+            SkDebugf("We do not have the data for image %d.\n", fid);
+            return nullptr;
+        }
+        return context->fImages[fid];
+    }
+    // Otherwise, the data is an image, deserialise it, store it in our map at its fid.
+    // TODO(nifong): make DeserialProcs accept sk_sp<SkData> so we don't have to copy this.
+    sk_sp<SkData> dataView = SkData::MakeWithCopy(data, length);
+    const sk_sp<SkImage> image = SkImage::MakeFromEncoded(std::move(dataView));
+    context->fImages.push_back(image);
+    return image;
+}