Fix extension browser action webview displaying no content when first opened

[petemill]

Fix #13633 via a workaround to hide and show the webview after the first time it is attached to a new WebContents.
Also avoid flash of different sizes by only setting a new size once the page has finished load (success or fail).

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.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

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

[bsclifton]

Tested with both bitwarden and lastpass- works great 😄

Did you want to capture specific test steps and link to them in the issue? (then you can label w/ the release notes include / test steps included)

[bsclifton]

master e23baf9
0.24.x 0e1aa80
0.23.x e1944b3