Force RefPtr::get() to be inline

For some reason GCC does not (always) inline 'RefPtr::get()'
and Intel VTune Amplifier profiler finds it to be a hotspot.
For example when content_shell is launched with '--dump-render-tree'
option for "fast/css" layout tests this method (in particular
WTF::RefPtr<WebCore::StyleSurroundData>::get) eats 8ms from
700ms of total CPU time (tested on Linux desktop x64).

This patch solves the problem.

Review URL: https://codereview.chromium.org/166463002

git-svn-id: svn://svn.chromium.org/blink/trunk@167182 bbb929c8-8fbe-4397-9dbb-9b2b20218538