Don't attempt to fill ContentBlock unless Content-Length > 0.
Don't attempt to fill ContentBlock unless Content-Length > 0.

When content with a Content-Length of 0 comes in the code incorrectly
assumes there's more data to be read.

While it's a bug that we have Content-Length 0 data coming in, the
harness shouldn't explode with an obscure ValueError in this case.

BUG=232044
TEST=android layout tests don't fail when empty PNG images come in.

Review URL: https://chromiumcodereview.appspot.com/16997007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152835 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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