unit_tests failing on multiple builders

Builders failed on: 
- linux-chromeos-dbg: 
  https://build.chromium.org/p/chromium.chromiumos/builders/linux-chromeos-dbg
- Linux Tests (dbg)(1)(32): 
  https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29
- Mac10.9 Tests (dbg): 
  https://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests%20%28dbg%29

Example broken build:
https://ci.chromium.org/buildbot/chromium.linux/Linux%20Tests%20%28dbg%29%281%29%2832%29/46140

Clearly caused by introduction of ExternalProtocolHandlerTest.TestUrlEscape in r518848.