Floating-point calculations are not guaranteed to happen at compile time
unless you force the issue with constexpr. This initializer was found
by running tools\win\static_initializers on a canary build
chrome_child.dll. constexpr was added to kSilenceRms for consistency.
BUG=chromium:341941
Review-Url: https://codereview.webrtc.org/2943833002
Cr-Commit-Position: refs/heads/master@{#18684}