These changes bring our copy of devtools_ui.cc in line with
https://chromium.googlesource.com/chromium/src.git/+/43.0.2357.92/chrome/browser/ui/webui/devtools_ui.cc
Without these changes, opening the devtools prints errors like:
[0602/165604:ERROR:CONSOLE(0)] "Refused to display 'chrome-devtools://devtools/inspector.html?can_dock=true&toolbarColor=rgba(223,223,223,1)&textColor=rgba(0,0,0,1)&experiments=true' in a frame because it set 'X-Frame-Options' to 'DENY'.", source: about:blank (0)
[0602/165604:ERROR:CONSOLE(1514)] "Uncaught SecurityError: Sandbox access violation: Blocked a frame at "chrome-devtools://devtools" from accessing a frame at "null". The frame being accessed is sandboxed and lacks the "allow-same-origin" flag.", source: chrome-devtools://devtools/devtools.js (1514)
