We have many duplicate code between content and chrome MediaBrowserTest. The same for EncryptedMediaTest. 

Today they share the same set of js/html, but the C++ handling is duplicated. The C++ code diverges over time, sometimes introducing bugs.

For example, in chrome, kError is "error", with an explanation and TODO to optimize it [1]. But in content, kError is "ERROR" [2], which is a bug.

I think some common helper/util file/class would make some sense so that the two tests can share the same mechanism how to interact with js/html.

[1] https://cs.chromium.org/chromium/src/chrome/browser/media/media_browsertest.cc?rcl=0&l=28

[2] https://cs.chromium.org/chromium/src/content/browser/media/media_browsertest.cc?rcl=0&l=24