Fix performance regression in PerformanceTests/DOM/modify-element-title

The change https://codereview.chromium.org/52093002 caused a regression
of about 7% in a couple of tests (see linked bug). This patch fixes the
regression by forcing a function that gcc would previously inline to be
inlined.

I believe this is as adding extra HashMaps to CSSAnimationUpdate caused
its destructor to become larger and as a result the compiler decided to
not inline the setAttributeInternal function. Without the patch causing
the regression, adding a single unused CompositableValueMap didn't make
any difference, while adding two would reproduce the regression.

BUG=315100

Review URL: https://codereview.chromium.org/61583002

git-svn-id: svn://svn.chromium.org/blink/trunk@161700 bbb929c8-8fbe-4397-9dbb-9b2b20218538