Add null check in ChromeClientImpl::postAccessibilityNotification().

Because AXObjectCache::accessibilityEnabled() always returns true in the shared worker process,
ChromeClientImpl::postAccessibilityNotification() is never called in the process.
So we don't need to check m_webView->client() now.

But we are planing to move SharedWorker to the renderer process.
http://crbug.com/327256
ChromeClientImpl::postAccessibilityNotification() will be called in the shared worker's WebFrame
when the accessibility feature is enabled.
But m_webView->client() is NULL in the shared worker's WebFrame.

So we have to check it.

BUG=327256

Review URL: https://codereview.chromium.org/134643019

git-svn-id: svn://svn.chromium.org/blink/trunk@165606 bbb929c8-8fbe-4397-9dbb-9b2b20218538