Introduce SpellChecker class for decoupling Editor and SpellChecker

This patch introduces SpellChecker class extracted from Editor class for ease of maintenance.

BUG=n/a
TEST=n/a; no behavior changes, exiting tests cover this changes.
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26565013

git-svn-id: svn://svn.chromium.org/blink/trunk@159318 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make EventPathWalker calculate adjusted event targets.

There are several places where adjusted event targets are calculated. That's bad since the code are very similar.
This patch makes EventPathWalker calculate adjusted event targets so that users of EventPathWalker can use the result.

This patch is separated from https://codereview.chromium.org/26191003/.

BUG=None
TEST=None; No behavior changes;

Review URL: https://codereview.chromium.org/26425004

git-svn-id: svn://svn.chromium.org/blink/trunk@159319 bbb929c8-8fbe-4397-9dbb-9b2b20218538