Assertions for TreeShared::m_deletionHasBegun should be ASSERT_WITH_SECURITY_IMPLICATION.

They are typical signals for use-after-free.

The main changes are conversions from ASSERT(!m_deletionHasBegun) to
ASSERT_WITH_SECURITY_IMPLICATION(!m_deletionHasBegun).

Declaration of m_deletionHasBegun and code to update m_deletionHasBegun should
be enabled only if ADDRESS_SANITIZER is defined or assertions are enabled. This
CL introduces SECURITY_ASSERT_ENABLED to represent this condition.

Some assertion-related code were wrongly wrapped with !NDEBUG. It should be
!ASSERT_DISABLED.

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/26689006

git-svn-id: svn://svn.chromium.org/blink/trunk@159284 bbb929c8-8fbe-4397-9dbb-9b2b20218538