Make sure AllowRepaintScope::m_originalValue is always initialized

Make sure AllowRepaintScope::m_originalValue is always initialized. This fixes a
build error with gcc 4.8:
"""
core/frame/FrameView.h:567:65: error: ‘*((void*)& scoper +8)’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
"""

R=abarth@chromium.org, abarth, eseidel

Review URL: https://codereview.chromium.org/151483012

git-svn-id: svn://svn.chromium.org/blink/trunk@166885 bbb929c8-8fbe-4397-9dbb-9b2b20218538