Fixes breaking amount input field on Windows and Linux

[ryanml]

Fixes brave/brave-browser#11958

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).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint, npm run gn_check)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.
• Added appropriate QA labels (QA/Yes or QA/No) to the associated issue
• Added appropriate release note labels (release-notes/include or release-notes/exclude) to the associated issue
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Defined in issue

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[emerick]

LGTM

[srirambv]

Verification passed on

Brave	1.17.11 Chromium: 86.0.4240.68 (Official Build) nightly (64-bit)
Revision	ad72ee9aa8e15ed300df1238e76c7a8f4d686f97-refs/branch-heads/4240@{#1097}
OS	Linux

• Verified input field with dropdown is properly aligned
• Verified changing the browser window size doesn't affect the input field size
  

---

Verification passed on

Brave	1.17.11 Chromium: 86.0.4240.68 (Official Build) nightly (64-bit)
Revision	ad72ee9aa8e15ed300df1238e76c7a8f4d686f97-refs/branch-heads/4240@{#1097}
OS	Windows 10 OS Version 1809 (Build 17763.1457)

• Verified input field with dropdown is properly aligned
• Verified changing the browser window size doesn't affect the input field size
  

---

Verification passed on

Brave	1.17.11 Chromium: 86.0.4240.68 (Official Build) nightly (x86_64)
Revision	ad72ee9aa8e15ed300df1238e76c7a8f4d686f97-refs/branch-heads/4240@{#1097}
OS	macOS Version 10.15.7 (Build 19H2)

• Verified changes didn't break the input field on macOS on restore/maximized window