Fixing magic number bug in downloads bar

[maazadeeb]

• 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).

Test Plan:

Fixes #3604

Added a new variable for downloads bar height in variables.less and consumed this across the codebase where 50px was hardcoded for this.

Auditors: @bsclifton

[bsclifton]

I pulled the code locally and manually tested each of the scenarios (as-is with 50px and then setting to another height):

1. The actual download bar itself
2. Positioning of the autofill context menu when filling a form
3. Positioning of the username context menu (when using built-in pwd mgr)
4. Title that shows in bottom left when doing a mouse over for an anchor tag

We're wrapping up our 0.12.5 release, this can go into our next release 0.12.6

PR is ready for merge. Thanks, @maaz93! 😄

[maazadeeb]

@bsclifton Thanks! This would be my first open source contribution! I will continue fixing more bugs to understand the codebase more 😄

[bbondy]

great cleanup @maaz93, thanks a ton!
When the downloads bar was first added, chromium didn't have support for css variables like this.

[maazadeeb]

You're welcome @bbondy ! Really happy you liked the clean up! 😄