Inline FloatSize::FloatSize(WebCore::IntSize const&) constructor.

The 'FloatSize::FloatSize(WebCore::IntSize const&)' invoked
from 'WebCore:ScrollView::visibleContentRect()' is mentioned
as a hotspot by the Intel VTune Amplifier profiler when
running 'Bindings/scroll-top.html' performance test.

This patch drives 'FloatSize::FloatSize(WebCore::IntSize const&)'
out from hotspot list, the profiler shows the 17.266s - 16.892s = 0.374s CPU time
gain with the mentioned performance test (linux desktop x64,
GCC 4.6.3, release content_shell build with enabled 'linux_dump_symbols' and 'use_aura' flags).

The results of the test  itself shows approx. 2% improvement.

before:
avg 179.5117899549199 runs/s
median 179.34441817013072 runs/s
stdev 0.961589016681394 runs/s
min 177.50819073435397 runs/s
max 180.8325271219252 runs/s

after:
avg 182.7270546104547 runs/s
median 183.16105219039957 runs/s
stdev 1.6022086143321321 runs/s
min 178.72507742693386 runs/s
max 185.07648946767654 runs/s

Review URL: https://codereview.chromium.org/174163011

git-svn-id: svn://svn.chromium.org/blink/trunk@167762 bbb929c8-8fbe-4397-9dbb-9b2b20218538