Fix newtab button isn't clickable

[simonhong]

With chromium's recent change, this button's border path rect is
calculated by using corder radius with layout value of EMPHASIS_MAXIMUM.
However, we changed that value to 4. Because of this, path rect is
calculated much smaller than buttons size.
So, most hittest was failed.

This also fixes the problem that a blob is visible on newtab button when theme is installed.
The blob's rect is also same as above small clickable rect.
W/o theme, button background and tab strip background color is same.
So, we didn't see that blob but it was there.

Close brave/brave-browser#1092
Close brave/brave-browser#983

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.
• Add appropriate QA labels (QA/Needed or QA/No-QA-Needed) to include the closed issue in milestone

Test Plan:

Check new tab button work with mouse click

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

[jasonrsadler]

Comments left. Looking forward to this being fixed :)

[petemill]

LGTM - Thanks for the fix! I knew it was an issue with themes, but guessing it was made worse with C70 to affect the hit test.

Updated

[petemill]

0.55.x 04a458f

[bbondy]

Updated both issues milestones. One wasn't set and the other was set wrongly to 1.0.