Prepare SVGLengthContext for oilpan

SVGLengthContext is made STACK_ALLOCATED, as it is currently never used
on a heap.

BUG=357163

Review URL: https://codereview.chromium.org/339793004

git-svn-id: svn://svn.chromium.org/blink/trunk@176268 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: Trace DocumentVisibilityObserver::m_document.

BUG=357163

Review URL: https://codereview.chromium.org/336293002

git-svn-id: svn://svn.chromium.org/blink/trunk@176269 bbb929c8-8fbe-4397-9dbb-9b2b20218538