Use TRACE category 'input' in WebViewImpl

It's often important to trace input events across components (browser, webkit,
etc.) and so elsewhere we are already using a common 'input' category.  This
makes WebViewImpl consistent with this pattern.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/19256019

git-svn-id: svn://svn.chromium.org/blink/trunk@154427 bbb929c8-8fbe-4397-9dbb-9b2b20218538