Before reporting coverage data, we attempt to reduce clutter by
merging nested and consecutive ranges. Nested ranges are merged, if
the child range has the same execution count as the parent range.
Sibling ranges are merged, if one sibling begins where the other ends
and execution counts are identical.

This allowed an invalid transformation in which a range with an
execution count of 1 would be merged into the parent change, but the
sibling range with identical start and end points and a count of 0
would remain, effectively deleting the covered range.

For example:

{start: 0, end: 10, count: 1},
{start: 5, end:  8, count: 1},  // It's invalid to remove this.
{start: 5, end:  8, count: 0}

The fix is to separate the parent and sibling merge passes, and
removing duplicate ranges in-between.

Bug: chromium:827530
Change-Id: Ic35eae1d4a106746570ce9cb412ed6710ef6da53
Reviewed-on: https://chromium-review.googlesource.com/992114Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52352}