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Uploading counterpart of http://crrev.com/124146. Provide a method for
the users of URLFetcher to obtain upload progress.

Different from the download case, even the internals of URLFetcher
(= URLRequest) doesn't provide callback mechanism for upload progress.
So the current implementation runs a timer and poll the progress status.

BUG=chromium-os:27370
TEST=content_unittests --gtest_filter='*URLFetcher*'

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=126589
Reverted: https://src.chromium.org/viewvc/chrome?view=rev&revision=126611
(XP test bots hung at URLFetcher test)

Review URL: https://chromiumcodereview.appspot.com/9618051

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@126850 0039d316-1c4b-4281-b951-d872f2087c98