Flush denormals to zero on arm and arm64.

We also slightly refactor the code to make it a little easier to read.

The impact of denormals on arm appear to be much less pronounced than
on x86, but flushing denormals will make arm and x86 results more similar.

This was tested manually by creating a test to check the response of a
lowpass filter to a very small impulse. All values after a certain
point should be zero when denormals are flushed to zero. If denormals
are not flushed, the non-zero output would last much longer.

Verified on Linux, Windows, OSX, and Android that denormals are flushed.

The test is not included; it seems not that important to ensure
denormals are flushed.

BUG=395254

Review URL: https://codereview.chromium.org/402803003

git-svn-id: svn://svn.chromium.org/blink/trunk@178620 bbb929c8-8fbe-4397-9dbb-9b2b20218538