Explicit PrefixSet test prefix distribution.

Explicit PrefixSet test prefix distribution.

Previously, the shared prefixes used a count which would (often) cause
multiple prefixes to end up within 2^16 of each other.  Modify the setup
to explicitly generate both denser and sparser clusters, to allow
reducing the overall number of prefixes required.

BUG=65151
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=238273

[Paweł Hajdan Jr., 2013-12-02 23:36:51]

LGTM, thank you. :)

[commit-bot: I haz the power, 2013-12-03 00:03:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shess@chromium.org/99883003/1

[commit-bot: I haz the power, 2013-12-03 02:30:13]

Change committed as 238273