Alias 4KB at the beginning of JSONParser::ConsumeDictionary() to help diagnose issue 791487.

R=siggi@chromium.org

Bug: 791487
Change-Id: I4535df5a52167f2f5b10424a07e035e1c46e8cfb
Reviewed-on: https://chromium-review.googlesource.com/850722
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527154}

base/json/json_parser.cc

@@ -8,6 +8,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/debug/alias.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/strings/string_number_conversions.h"
@@ -327,6 +328,16 @@ Optional<Value> JSONParser::ParseToken(Token token) {
 }
 
 Optional<Value> JSONParser::ConsumeDictionary() {
+  // Attempt to alias 4KB of the buffer about to be read. Need to alias multiple
+  // sites as the crashpad heuristics only grab a few hundred bytes in
+  // front/behind heap pointers on the stack.
+  // TODO(gab): Remove this after diagnosis of https://crbug.com/791487 is
+  // complete.
+  const char* initial_pos[16];
+  for (size_t i = 0; i < arraysize(initial_pos); ++i)
+    initial_pos[i] = pos_ + i * 256;
+  debug::Alias(&initial_pos);
+
   if (*pos_ != '{') {
     ReportError(JSONReader::JSON_UNEXPECTED_TOKEN, 1);
     return nullopt;