Simplify API::PolicyClient management

Simplify API::PolicyClient management
https://bugs.webkit.org/show_bug.cgi?id=127529

Reviewed by Tim Horton.

Add default implementations to API::PolicyClient and change WebPageProxy::setPolicyClient to
always create a policy client, even if the policy object set is null. That way we don't have to null
check the policy client everywhere.

* UIProcess/API/APIPolicyClient.h:
(API::PolicyClient::decidePolicyForNavigationAction):
(API::PolicyClient::decidePolicyForNewWindowAction):
(API::PolicyClient::decidePolicyForResponse):
(API::PolicyClient::unableToImplementPolicy):
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::setPolicyClient):
(WebKit::WebPageProxy::decidePolicyForNavigationAction):
(WebKit::WebPageProxy::decidePolicyForNewWindowAction):
(WebKit::WebPageProxy::decidePolicyForResponse):
(WebKit::WebPageProxy::unableToImplementPolicy):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@162672 268f45cc-cd09-0410-ab3c-d52691b4dbfc