[fuzzer] Add input validation in the beginning of the parser fuzz target.

Non-printable characters do not make sense.
Inputs with non balanced brackets are mostly useless as well.

This validation function makes the fuzzer 15-20x faster.

Also use -only_ascii=1 option of libFuzzer:
https://codereview.chromium.org/2875933003

BUG=chromium:584819

Review-Url: https://codereview.chromium.org/2881583002
Cr-Commit-Position: refs/heads/master@{#45367}

test/fuzzer/parser.cc

@@ -14,7 +14,51 @@
 #include "src/parsing/preparser.h"
 #include "test/fuzzer/fuzzer-support.h"
 
+#include <cctype>
+#include <list>
+
+bool IsValidInput(const uint8_t* data, size_t size) {
+  std::list<char> parentheses;
+  const char* ptr = reinterpret_cast<const char*>(data);
+
+  for (size_t i = 0; i != size; ++i) {
+    // Check that all characters in the data are valid.
+    if (!(std::isspace(ptr[i]) || std::isprint(ptr[i]))) {
+      return false;
+    }
+
+    // Check balance of parentheses in the data.
+    switch (ptr[i]) {
+      case '(':
+      case '[':
+      case '{':
+        parentheses.push_back(ptr[i]);
+        break;
+      case ')':
+        if (parentheses.back() != '(') return false;
+        parentheses.pop_back();
+        break;
+      case ']':
+        if (parentheses.back() != '[') return false;
+        parentheses.pop_back();
+        break;
+      case '}':
+        if (parentheses.back() != '{') return false;
+        parentheses.pop_back();
+        break;
+      default:
+        break;
+    }
+  }
+
+  return parentheses.empty();
+}
+
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  if (!IsValidInput(data, size)) {
+    return 0;
+  }
+
   v8_fuzzer::FuzzerSupport* support = v8_fuzzer::FuzzerSupport::Get();
   v8::Isolate* isolate = support->GetIsolate();