[ChromeDriver] Fix takeHeapSnapshot command

Return heap snapshot as a string instead of JSON object.
This resolves two issues:
* The snapshot sometimes uses characters not allowed by our JSON parser.
* Chrome's snapshot reader is sensitive to the ordering of properties
  in the snapshot. Converting the snapshot into JSON object and then
  re-serialize into string often results in a file not loadable by the
  snapshot reader.

Bug: chromedriver:2964, chromedriver:1616
Change-Id: Ia721c28ed78a085d567111459c9e50d8bc26f81d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679224Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tatiana Buldina <buldina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672666}

chrome/test/chromedriver/chrome/heap_snapshot_taker.cc

@@ -28,13 +28,7 @@ Status HeapSnapshotTaker::TakeSnapshot(std::unique_ptr<base::Value>* snapshot) {
 
   Status status3(kOk);
   if (status1.IsOk() && status2.IsOk()) {
-    std::unique_ptr<base::Value> value =
-        base::JSONReader::ReadDeprecated(snapshot_);
-    if (!value) {
-      status3 = Status(kUnknownError, "heap snapshot not in JSON format");
-    } else {
-      *snapshot = std::move(value);
-    }
+    *snapshot = std::make_unique<base::Value>(std::move(snapshot_));
   }
   snapshot_.clear();
   if (status1.IsError()) {

chrome/test/chromedriver/chrome/heap_snapshot_taker_unittest.cc

@@ -22,10 +22,8 @@ namespace {
 const char* const chunks[] = {"{\"a\": 1,", "\"b\": 2}"};
 
 std::unique_ptr<base::Value> GetSnapshotAsValue() {
-  std::unique_ptr<base::DictionaryValue> dict(new base::DictionaryValue());
-  dict->SetInteger("a", 1);
-  dict->SetInteger("b", 2);
-  return std::move(dict);
+  std::string str_snapshot = "{\"a\": 1,\"b\": 2}";
+  return std::make_unique<base::Value>(std::move(str_snapshot));;
 }
 
 class DummyDevToolsClient : public StubDevToolsClient {