This change removes base::Value::CreateNullValue in favor of Value's default constructor. In particular, this change

- Replaces |Value::CreateNullValue()| with |MakeUnique<Value>()|.
  - Adds #includes of base/memory/ptr_util.h where needed.
  - Replaces |std::unique_ptr<Value>| with |auto| where appropriate.
- Replaces |*Value::CreateNullValue()| with |Value()|.
- Replaces |Value::CreateNullValue().release()| with |new Value()|.

BUG=646113
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2792573002
Cr-Commit-Position: refs/heads/master@{#462794}