Fix tests to avoid page scale reset when setting page scale.

This CL fixes an error that breaks layout tests when an default override scale is specified for desk top. (See https://chromiumcodereview.appspot.com/17830003/). It also combines eventSender.scalePageBy and internals.setPageScaleFactor into a single call-location, eventSender.setPageScaleFactor.

BUG=162482

Review URL: https://chromiumcodereview.appspot.com/18546003

git-svn-id: svn://svn.chromium.org/blink/trunk@155784 bbb929c8-8fbe-4397-9dbb-9b2b20218538