Fix lsan failure with ManifestParserTest.*

BUG=None
TBR=rockot@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/554043004

Cr-Commit-Position: refs/heads/master@{#294870}

content/renderer/manifest/manifest_parser.cc

@@ -54,7 +54,7 @@ base::NullableString16 ParseShortName(
 namespace content {
 
 Manifest ManifestParser::Parse(const base::StringPiece& json) {
-  base::Value* value = base::JSONReader::Read(json);
+  scoped_ptr<base::Value> value = base::JSONReader::Read(json);
   if (!value) {
     // TODO(mlamouri): get the JSON parsing error and report it to the developer
     // console.