Rename variables and private functions to follow style,
replace remaining asserts with DCHECKs.
add 'ms' suffix to time variables derived from clock_
add 'ntp' suffix to time variables derived from ntp time.
No functional changes expected.
BUG=None
Review-Url: https://codereview.webrtc.org/2588753002
Cr-Commit-Position: refs/heads/master@{#15706}