Increase urlbarForm input height to the equal value as 'urlbarFormHeight'

[luixxiul]

Also:

• Set white background to .urlbarForm inside navigator
• Remove redundant specifications (the specified background is inherited to all of the elements inside .urlbarForm)
• Remove duplicate variables

Fixes #4922
Addresses #9283

Auditors:

Test Plan:

1. Make sure that clicking the area between the top and bottom edge of the URL bar selects the URL address

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

Assigning @cezaraugusto & @bsclifton for review - it would be appreciated if one of you could check if the fix should work as expected on Windows.

[luixxiul]

I know that making one tiny change on navigationBar.less (which is messy to be honest) could lead to visual regressions, while this case I think it would not.

I will start working on that file soon, after browserButton is mostly refactored.

[cezaraugusto]

++