fix browser setup for Chrome (#857)

[apastel]

Fixes #857

Supports all three of the following formats for pasting headers:

1. Firefox
2. Chrome (new format without colons)
3. Chrome (old format with colons on header keys)

Had to add a corner case to handled the odd Decoded: header in Chrome, which looks like the following:

Decoded:
message ClientVariations {
  // Active Google-visible variation IDs on this client. These are reported for analysis, but do not directly affect any server-side behavior.
  repeated int32 variation_id = [3300102, 3300132, 3313321, 3323055, 3330197, 3362821, 3393046, 3393674, 3395745, 3396053, 3396249, 3396352, 3396365, 3396446, 3396569, 3396819, 3396954, 3397042, 3397046];
  // Active Google-visible variation IDs on this client that trigger server-side behavior. These are reported for analysis *and* directly affect server-side behavior.
  repeated int32 trigger_variation_id = [3392236, 3396349];
}

It's the only one that still has a colon on the header key so it had to be handled differently. The end result in the parsed user_headers is the same as it was before, where the property ends up being Decoded: "}"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.51%. Comparing base (5578f89) to head (149b4dd).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #865   +/-   ##
=======================================
  Coverage   95.51%   95.51%           
=======================================
  Files          46       46           
  Lines        2543     2543           
=======================================
  Hits         2429     2429           
  Misses        114      114           

Flag	Coverage Δ
unittests	95.51% <ø> (ø)

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