Telemetry: Fix precision problem in slice ends.
Telemetry: Fix precision problem in slice ends.

The epsilon being used was too small for the large
microsecond values in some traces. This rounds back to the
source precision which should be robust (up to double
precision limits for storing micros-as-millis).

BUG=335833

Review URL: https://codereview.chromium.org/144743002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@246513 0039d316-1c4b-4281-b951-d872f2087c98

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