Fix check for user gesture on password autofill

The original security feature is:
If passwords get autofilled on page load, do not make them accessible to JavaScript code until the user interacts with the page.

Formerly, this was implemented as https://codereview.chromium.org/83023017 (Chromium) + https://codereview.chromium.org/82693006 (Blink). That implementation used the Blink user gesture support in a wrong way (https://codereview.chromium.org/82693006/#msg14), which resulted in http://crbug.com/337429.

This Cl attempts to fix the problem by going through RenderWidget::OnHandleInputEvent. At that point we can see user events and filter them using WebInputEvent::isUserGestureEventType. Through RenderViewObserver, we then notify the corresponding PasswordAutofillAgent.

Testing:
This CL adds a test replicating the problem from http://crbug.com/337429. vabr checked that the new test fails without the new fix.
All the tests put in place by https://codereview.chromium.org/83023017 are also relevant.
vabr also manually checked with the browser, that the issue reported in http://crbug.com/337429 no longer reproduces with this CL.

BUG=337429,163072

Review URL: https://codereview.chromium.org/163843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252352 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252352