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This test turns off caching for the reload request initiated via the DevTools network agent.
Without PlzNavigate, this goes through the normal resource load path, where in
RenderFrameImpl::decidePolicyForNavigation is invoked after which the cache policy is set in
the WebURLRequest to disabled correctly.

With PlzNavigate, the reload request is sent out to the browser via the BeginNavigate IPC
in RenderFrameImpl::decidePolicyForNavigation() and we indicate  to blink that the client
handled the request.

As a result the cache policy does not get set to disabled.

Fix is to add query the dev tools agent if caching is disabled. To achieve this following changes
were needed.
1. Add a virtual function cacheDisabled() to the WebDevToolsAgent class.
2. This function calls to the newly added cacheDisabled() function in the InspectorNetworkAgent class.
   which checks the state variable for the same.

BUG=673745

Review-Url: https://codereview.chromium.org/2611183007
Cr-Commit-Position: refs/heads/master@{#442773}