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This is a reland of 107694e1

The change was reverted due to an uninitialized member on an Impression
object in the test. The reland defaults Impression.impression_data to
0 to avoid these types of errors.

Original change's description:
> [conversions] Use base 10 encoding for impression and conversion data
>
> What:
> No longer parse impressiondata as hex, or use hex when sending
> conversion reports.
>
> Why:
> This causes needless conversions for developers, as raised on
> https://github.com/WICG/conversion-measurement-api/issues/60
>
> Note that the conversions database stores imp and conversion data as
> strings currently because they are represented as hex. These can
> be converted to strings now, but this is left as a followup.
>
> Bug: 1163599
> Change-Id: I5b164c380a97cb272c6a0ee698df8b744feb58ae
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611958
> Reviewed-by: Charlie Harrison <csharrison@chromium.org>
> Reviewed-by: Nate Chapin <japhet@chromium.org>
> Commit-Queue: John Delaney <johnidel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#842561}

Bug: 1163599
Change-Id: I192ba4b9452c3ad97f9511ce093f36f9c697ba2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625293Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843143}