Clean EventHandler::dispatchMouseEvent up.

- Rename a local value 'swallowEvent' to 'proceedDefault,' and negate
   the value in order to match the return value.
- Removes unused 'cancelable' argument.

BUG=360454
TEST=none; should not have any behavior changes.

Review URL: https://codereview.chromium.org/227343003

git-svn-id: svn://svn.chromium.org/blink/trunk@171113 bbb929c8-8fbe-4397-9dbb-9b2b20218538