Cleanup: Start using toFoo methods as part of newly adopted coding guideline.

As part of newly adopted coding guidelines, static_cast<Classname*> is discouraged and toFoo calling is encouraged. To enforce the new guideline, a patch (https://codereview.chromium.org/23654034/) on path to landing, which would raise presubmit error if it finds use of static_cast<Classname*> in the patch. This patch replaces static_cast<Classname*> in present codebase with toFoo(), at all places for which toFoo() is already defined.

R=tkent, inferno, abarth

BUG=None

TEST=None; no behavior change; No regressions with layout tests, webkit_unit_tests, content_unittests

Review URL: https://chromiumcodereview.appspot.com/24253004

git-svn-id: svn://svn.chromium.org/blink/trunk@158094 bbb929c8-8fbe-4397-9dbb-9b2b20218538