Tell V8 about the extra memory being held by FileReader objects.

third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp

Index: third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
diff --git a/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp b/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
index d0e1b24c2d903d35d7c45a57fef9bfe7281ea8a8..653ccd9cc0cbd35c8929a76182abf8fe53460e4a 100644
--- a/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
+++ b/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
@@ -50,6 +50,7 @@
 #include "platform/wtf/text/Base64.h"
 #include "platform/wtf/text/StringBuilder.h"
 #include "public/platform/WebURLRequest.h"
+#include "v8/include/v8.h"
 
 namespace blink {
 
@@ -69,6 +70,7 @@ FileReaderLoader::FileReaderLoader(ReadType read_type,
 
 FileReaderLoader::~FileReaderLoader() {
   Cleanup();
+  UnreportMemoryUsageToV8();
   if (!url_for_reading_.IsEmpty()) {
     BlobRegistry::RevokePublicBlobURL(url_for_reading_);
   }
@@ -143,9 +145,23 @@ void FileReaderLoader::Cleanup() {
     string_result_ = "";
     is_raw_data_converted_ = true;
     decoder_.reset();
+    UnreportMemoryUsageToV8();
   }
 }
 
+void FileReaderLoader::ReportAdditionalMemoryUsageToV8(int64_t usage) {
+  memory_usage_reported_to_v8_ += usage;
+  v8::Isolate::GetCurrent()->AdjustAmountOfExternalAllocatedMemory(usage);
+}
+
+void FileReaderLoader::UnreportMemoryUsageToV8() {
+  if (!memory_usage_reported_to_v8_)
+    return;
+  v8::Isolate::GetCurrent()->AdjustAmountOfExternalAllocatedMemory(
+      -memory_usage_reported_to_v8_);
+  memory_usage_reported_to_v8_ = 0;
+}
+
 void FileReaderLoader::DidReceiveResponse(
     unsigned long,
     const ResourceResponse& response,
@@ -230,6 +246,7 @@ void FileReaderLoader::DidReceiveData(const char* data, unsigned data_length) {
   }
   bytes_loaded_ += bytes_appended;
   is_raw_data_converted_ = false;
+  ReportAdditionalMemoryUsageToV8(bytes_appended);
 
   if (client_)
     client_->DidReceiveData();
@@ -332,6 +349,8 @@ String FileReaderLoader::StringResult() {
       NOTREACHED();
   }
 
+  if (finished_loading_)
+    ReportAdditionalMemoryUsageToV8(string_result_.CharactersSizeInBytes());

[michaeln, 2017/04/26 20:36:59]

Hmmm... I think this can miss reporting the the string result in some cases
where the result is sample prior to load completion. I'm not sure how much
accuracy matters though?

We could dbl count the raw data bytes appended and not even try to count the
string_result_ independently. That would error on the side of over-reporting
rather than under-reporting which might be a better compromise. Assuming
somewhat over-reporting is not a problem, I think I'd prefer that to a bunch of
code to perform careful accounting for the string_result_.

wdyt?

   return string_result_;
 }