Specify values as nonnull in Selection API to align with the spec

This includes:
  - the return type of `Selection.prototype.getRangeAt`
  - the `range` parameter of `Selection.prototype.removeRange`
  - the `node` parameter of `Selection.prototype.containsNode`

The parameters in Selection.prototype.setBaseAndExtent have also be
updated to align with the names used in the spec.

This involved changing the return type to void and making the event
Non null.

Closes #3389.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=249925099

externs/browser/gecko_dom.js

@@ -485,7 +485,7 @@ Selection.prototype.addRange = function(range) {};
 
 /**
  * @param {number} index
- * @return {Range}
+ * @return {!Range}
  * @nosideeffects
  * @see https://w3c.github.io/selection-api/#dom-selection-getrangeat
  */
@@ -520,7 +520,7 @@ Selection.prototype.collapseToEnd = function() {};
 Selection.prototype.collapseToStart = function() {};
 
 /**
- * @param {Node} node
+ * @param {!Node} node
  * @param {boolean=} allowPartialContainment
  * @return {boolean}
  * @nosideeffects
@@ -549,7 +549,7 @@ Selection.prototype.extend = function(parentNode, offset) {};
 Selection.prototype.removeAllRanges = function() {};
 
 /**
- * @param {Range} range
+ * @param {!Range} range
  * @return {undefined}
  * @see https://w3c.github.io/selection-api/#dom-selection-removerange
  */

externs/browser/webkit_dom.js

@@ -108,15 +108,15 @@ Selection.prototype.type;
 Selection.prototype.empty = function() {};
 
 /**
- * @param {Node} baseNode
- * @param {number} baseOffset
- * @param {Node} extentNode
- * @param {number} extentOffset
+ * @param {Node} anchorNode
+ * @param {number} anchorOffset
+ * @param {Node} focusNode
+ * @param {number} focusOffset
  * @return {undefined}
  * @see https://w3c.github.io/selection-api/#dom-selection-setbaseandextent
  */
 Selection.prototype.setBaseAndExtent =
- function(baseNode, baseOffset, extentNode, extentOffset) {};
+ function(anchorNode, anchorOffset, focusNode, focusOffset) {};
 
 /**
  * @param {string} alter