Update to Chromium v61

[cztomczak]

Chromium v61 is currently supported Release branch in upstream CEF:
https://bitbucket.org/chromiumembedded/cef/wiki/BranchesAndBuilding#markdown-header-current-release-branches-supported

Breaking changes:

• Ubuntu 12 / 13 and Debian 7 no more supported by upstream Chromium/CEF (Issue Ubuntu 12 / 13 and Debian 7 no more supported in upstream Chromium/CEF #401)
• There is a breaking change in v59 in offscreen windows, they are now automatically transparent due to this CEF commit. To make a offscreen window non-transparent you need to set the background colour using BrowserSettings.background_color. Ref: Update to Cef Version 59.0.0 cefsharp/CefSharp#2114

New branch "cefpython58" should be created from current master before updating.

[cztomczak]

See Issue #403 ("Update to Chromium v62").