Declare kCachedPowersLength only if !ASSERT_DISABLED

The kCachedPowersLength global static variable is only used in assertions so
we should only declare it if !ASSERT_DISABLED to avoid warnings at compile
time.

R=eseidel, thakis
BUG=312287
NOTRY=true

Review URL: https://codereview.chromium.org/57433008

git-svn-id: svn://svn.chromium.org/blink/trunk@161295 bbb929c8-8fbe-4397-9dbb-9b2b20218538