s13n: Expose an API to get the total of decompressed bytes of a response body

While migrating some classes from URLFetcher to SimpleURLLoader,
it is useful to have the size of the resulting decompressed body.

In case of DownloadToString* calls, this is straightfoward:
the length to the string is the size.
However, in the case of DownloadTo{Temp}File calls, where only the
resulting file path is passed to the completion callback, having
an API to get the decompressed body size is useful. It also avoids
sync calls to access the file data.

This CL adds a SimpleURLLoader::GetContentSize API that provides such
value. When used with SimpleURLLloader::SetAllowPartialResults(true),
the API returns the total decompressed body size even in the case of
errors.

BUG=773295,844972

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ia8cdb3326b5d5c74276e384f966a7e98f61a8c46
Reviewed-on: https://chromium-review.googlesource.com/1167683
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581952}
3 files changed