Revert "Set SVGExternalDocumentCache::Entry::document_ eagerly in NotifyFinished()"

This reverts commit 590a8d43.

Reason for revert: Suspect cause of inspector-protocol/emulation/set-vision-deficiency.js failures: https://test-results.appspot.com/data/layout_results/linux-bfcache-rel/8184/blink_web_tests/layout-test-results/results.html

Original change's description:
> Set SVGExternalDocumentCache::Entry::document_ eagerly in NotifyFinished()
> 
> Currently, we lazily initialize document the first time it is requested
> *and* all of the data is loaded (https://crbug.com/1107442 fixed a case
> where we would incorrectly initialize the document by creating it before
> all of the data had been received).
> 
> It's much clearer to just initialize the document when all of the data
> is received, and have GetDocument() be a simple getter.
> 
> Change-Id: I830dca5cc953ca8467fd936f5c783db21cfce051
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316415
> Commit-Queue: Nate Chapin <japhet@chromium.org>
> Reviewed-by: Fredrik Söderquist <fs@opera.com>
> Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#804325}

TBR=fs@opera.com,japhet@chromium.org,kouhei@chromium.org

Change-Id: Ib838f8e42475020ceb83f8aa841d1ad077d705bd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393159Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804461}

third_party/blink/renderer/core/svg/svg_external_document_cache.cc

@@ -58,22 +58,23 @@ void SVGExternalDocumentCache::Entry::AddClient(Client* client) {
 
 void SVGExternalDocumentCache::Entry::NotifyFinished(Resource* resource) {
   DCHECK_EQ(GetResource(), resource);
-  DCHECK(!document_);
+  for (auto client : clients_) {
+    if (client)
+      client->NotifyFinished(GetDocument());
+  }
+}
 
-  const TextResource* text_resource = To<TextResource>(resource);
-  if (text_resource->HasData() &&
-      MimeTypeAllowed(text_resource->GetResponse())) {
+Document* SVGExternalDocumentCache::Entry::GetDocument() {
+  const TextResource* resource = To<TextResource>(GetResource());
+  if (!document_ && resource->IsLoaded() && resource->HasData() &&
+      MimeTypeAllowed(resource->GetResponse())) {
     document_ = XMLDocument::CreateSVG(
         DocumentInit::Create()
-            .WithURL(text_resource->GetResponse().CurrentRequestUrl())
+            .WithURL(resource->GetResponse().CurrentRequestUrl())
             .WithExecutionContext(context_.Get()));
-    document_->SetContent(text_resource->DecodedText());
-  }
-  for (auto client : clients_) {
-    if (client)
-      client->NotifyFinished(GetDocument());
+    document_->SetContent(resource->DecodedText());
   }
-  clients_.clear();
+  return document_.Get();
 }
 
 void SVGExternalDocumentCache::Entry::Trace(Visitor* visitor) const {

third_party/blink/renderer/core/svg/svg_external_document_cache.h

@@ -54,7 +54,7 @@ class CORE_EXPORT SVGExternalDocumentCache
     explicit Entry(ExecutionContext* context) : context_(context) {}
     ~Entry() override = default;
     void Trace(Visitor*) const override;
-    Document* GetDocument() { return document_.Get(); }
+    Document* GetDocument();
     const KURL& Url() const { return GetResource()->Url(); }
 
    private: