Fixes #7353. During refactoring the DropZone component, getInitialState method was removed. However, its call in resetDragState method was not removed which causes 2169 errors in 24 hour period (thanks @artpi for your work on JS error logging!).

In this PR, I propose a simple fix in which we reset state in resetDragState more verbosely by enumerating specific props and their initial values instead of calling some method. Props @aduth for this solution and help.

I also reuse the resetDragState method in onDrop event handler as the resetting functionality was unnecessarily duplicated there.

Testing Instructions

To be honest, I couldn't reproduce the issue the "normal" way. I couldn't get the mouseup event triggered by using Calypso (maybe you could, here are some steps).

Fortunately, we can create and fire events manually through Dev Console. So, firstly, let's reproduce the bug and then see if it's fixed:

1. Checkout master branch and open client/components/drop-zone/index.jsx in your favorite editor;
2. Comment/delete these lines (they prevent us from seeing the error with the manual event firing);
3. Load Calypso Media Library or just create a new post;
4. Open Dev Tools and copy paste the following code in the Console tab:

var event = document.createEvent("HTMLEvents");

event.initEvent("mouseup", true, true);

window.dispatchEvent(event);

1. Notice the this.getInitialState is not a function error;
2. Checkout this branch and do the same;
3. Is there still an error?

cc @aduth @ockham @gwwar @timmyc @artpi

Test live: https://calypso.live/?branch=fix/drop-zone-reset-drag-state