[logviewer] missing contents from large logs

I ran some fairly long-running builds on a test buildbot 0.9.6 instance, which create > 24000 log lines. Once the build is done, I scrolled around the logs and noticed that there are blank patches of missing lines. Copy+pasting across the blank patches simply skips from the first line before the blank patch to the first line after it.

Tested with linux 64 bit chrome 58.0.3029.81.