Inline methods of QuicBandwidth class. No functional change; not

Inline methods of QuicBandwidth class.  No functional change; not
flag-protected.

Since "B" in "BBR" stands for bandwidth, we've been using QuicBandwidth class
much more actively on the hot paths.  The overhead from calling QuicBandwidth
methods started to show up on profiles, so I've maked a CL to make them all
inlined, as it was done with QuicTime::Delta.

This removes the non-negativity check from the QuicBandwidth constructor where
it would previously result in a QUIC_BUG, since it's now constexpr.

Merge internal change: 156806926

R=rch@chromium.org
BUG=

[Ryan Hamilton, 2017-06-01 15:21:53]

lgtm