Fix "value possibly truncated" warnings on MSVC, net/base edition.

Fix "value possibly truncated" warnings on MSVC, net/base edition.

This also adjusts MapSystemError() to take a SystemErrorCode, which wasn't
strictly necessary to remove a warning but seemed more accurate.

BUG=81439
TEST=none

Committed: https://crrev.com/32c741528d6be56f7c8ec712a744f673d1b4343e
Cr-Commit-Position: refs/heads/master@{#305749}

[Peter Kasting, 2014-11-25 21:42:07]

pkasting@chromium.org changed reviewers:
+ rch@chromium.org

[Peter Kasting, 2014-11-25 21:42:08]

https://codereview.chromium.org/760703002/diff/20001/net/base/upload_file_ele...
File net/base/upload_file_element_reader.cc (right):

https://codereview.chromium.org/760703002/diff/20001/net/base/upload_file_ele...
net/base/upload_file_element_reader.cc:143:
callback.Run(static_cast<int>(result));
At first I wondered if this shouldn't mean OnSeekCompleted() should take an
Int64CompletionCallback, but after reflection, I think the cast is more correct;
if result < 0, we're returning an error code, not a file offset, and in that
case, we know the value will fit in an int; indeed, since we use int and not
int64 to hold "error codes" everywhere in the codebase, it's appropriate for the
caller of this function to pass a callback that takes an int to "record the
error code resulting from the seek".

[Ryan Hamilton, 2014-11-26 00:07:15]

lgtm

[Peter Kasting, 2014-11-26 00:13:14]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2014-11-26 00:13:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/760703002/20001

[commit-bot: I haz the power, 2014-11-26 01:04:09]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-26 01:05:43]

Patchset 2 (id:??) landed as
https://crrev.com/32c741528d6be56f7c8ec712a744f673d1b4343e
Cr-Commit-Position: refs/heads/master@{#305749}