[omnibox] Narrow AutocompleteInput fuzzer input

Change fuzzer to avoid data we know AutocompleteInput isn't designed
to handle.

Change-Id: I755db8375adf0f629af6a656d9494033cae5bb39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876551Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Kevin Bailey <krb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709181}

components/omnibox/browser/autocomplete_input_fuzzer.cc

@@ -10,6 +10,7 @@
 #include "base/at_exit.h"
 #include "base/i18n/icu_util.h"
 #include "base/strings/string16.h"
+#include "base/strings/utf_string_conversion_utils.h"
 #include "components/omnibox/browser/test_scheme_classifier.h"
 #include "third_party/metrics_proto/omnibox_event.pb.h"
 
@@ -23,10 +24,20 @@ struct IcuEnvironment {
 IcuEnvironment icu_env;
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  // Enforce a reasonable bound on what we believe it takes to trigger
+  // an error.
+  if (size > 4096)
+    return 0;
   // This fuzzer creates a random UTF16 string, for testing primarily against
   // AutocompleteInput::Parse().
   base::string16 s(reinterpret_cast<const base::string16::value_type*>(data),
                    size / sizeof(base::string16::value_type));
+  // Some characters are considered illegal and, while our code handles them
+  // fine, fuzzing runs with DCHECKs enabled which will trigger on them.
+  for (auto c : s) {
+    if (!base::IsValidCharacter(c))
+      return 0;
+  }
   AutocompleteInput input(s, metrics::OmniboxEventProto::OTHER,
                           TestSchemeClassifier());
   return 0;