Make the multipart dump reader more resillient to incorrect dumps

If the content shell browser process dies between the time the minidump
is written and the time it's turned into a multipart file, the dump
reader will fail to read the dump and throw an exception instead.

BUG=none
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/142323007

git-svn-id: svn://svn.chromium.org/blink/trunk@165816 bbb929c8-8fbe-4397-9dbb-9b2b20218538