Make narrowPrecisionToFloat() use clampTo<float>() instead of static_cast.

For numbers within the range of valid floats, this has no effect.  For numbers
outside that range, the old code was technically undefined behavior (and in
practice would result in bad things like converting to infinity).  Now such
numbers will be deterministically clamped to +/-FLOAT_MAX.

BUG=416991
TEST=svg/text/svg-zoom-large-value.xhtml should not cause asserts with the patch on https://codereview.chromium.org/521543002/

Review URL: https://codereview.chromium.org/596913003

git-svn-id: svn://svn.chromium.org/blink/trunk@182563 bbb929c8-8fbe-4397-9dbb-9b2b20218538