Force inline of StringImpl::deref(ref) methods

The Intel VTune Amplifier profiler finds the
'StringImpl::deref' method as a hotspot when
running some of performance tests (Linux desktop
x64), meaning that GCC does not inline the method
even though it is expected to do so.

This patch forces inline of the 'StringImpl::ref',
'StringImpl::deref' and 'StringImpl::hasOneRef'
methods.

The performance gain can be observed with the
'DOM/modify-element-id.html' performance test:

Without the patch:
avg 106.57625000021653 ms
median 106.59550000036688 ms
stdev 3.633147553468632 ms
min 101.28500000064378 ms
max 111.95000000043365 ms

After the patch is applied:
avg 101.04664999989835 ms
median 99.81650000008813 ms
stdev 3.424950084772838 ms
min 96.42199999871082 ms
max 107.19700000117882 ms

Performance gain for the average value is approx. 5.5%.

The profiler shows the following delta of the total CPU
time used: 3.000s - 2.810s = 0.190s.

Review URL: https://codereview.chromium.org/170573002

git-svn-id: svn://svn.chromium.org/blink/trunk@167351 bbb929c8-8fbe-4397-9dbb-9b2b20218538