Replace RemoveWithoutPathExpansion(.*, nullptr) with Value::RemoveKey()

DictionaryValue::RemoveWithoutPathExpansion() is deprecated. For the
case where nullptr is passed for the output argument, Value::RemoveKey()
is a trivial replacement.

Mostly done with the following command line, plus a bit of finesse:
% git grep -l RemoveWithoutPathExpansion | \
  xargs sed -E -i 's/RemoveWithoutPathExpansion\((.*), nullptr/RemoveKey\(\1/g'

Bug: 646113

This CL was uploaded by git cl split.

R=rdevlin.cronin@chromium.org

Change-Id: I9bb8090747407e77408a9fa1556ccba3633cbd51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298783
Auto-Submit: Anand K Mistry <amistry@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788750}

tools/json_schema_compiler/test/crossref_unittest.cc

@@ -66,7 +66,7 @@ TEST(JsonSchemaCompilerCrossrefTest, TestTypeOptionalParamFail) {
   auto params_value = std::make_unique<base::ListValue>();
   std::unique_ptr<base::DictionaryValue> test_type_value =
       CreateTestTypeValue();
-  test_type_value->RemoveWithoutPathExpansion("number", NULL);
+  test_type_value->RemoveKey("number");
   params_value->Append(std::move(test_type_value));
   std::unique_ptr<crossref::TestTypeOptionalParam::Params> params(
       crossref::TestTypeOptionalParam::Params::Create(*params_value));