Gardening: The following tests are flaky.

- webmidi/requestmidiaccess-basic.html
- fast/events/menu-keydown-on-hidden-element.html
- http/tests/security/contentSecurityPolicy/report-only-from-header.php
- inspector/debugger/live-edit.html

BUG=282079,282080,282081,282082
TBR=rjkroege@chromium.org

Review URL: https://codereview.chromium.org/23543012

git-svn-id: svn://svn.chromium.org/blink/trunk@156953 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce toSVGTextElement(), and use it

As a step to clean-up static_cast<SVGXXX>, toSVGTextElement() is added to clean-up
static_cast<SVGTextElement*>.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23532011

git-svn-id: svn://svn.chromium.org/blink/trunk@156955 bbb929c8-8fbe-4397-9dbb-9b2b20218538