Consider brave theme type to devtools' theme

[simonhong]

When devtool ui is loaded, kDevToolsPreferences pref values are fetched.
In that dictionary prefs, uiTheme represents theme type(light/dark) of devtool.
To apply brave theme type to devtools, uiTheme value is replaced with brave
native theme type. If user sets devtools' theme type explicitely, that value
is used instead of brave theme type. Otherwise, brave theme type is applied.

Fix brave/brave-browser#784

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
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• 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/Yes or QA/No) to include the closed issue in milestone

Test Plan:

yarn test brave_browser_tests --filter=BraveDevToolsUIBindingsBrowserTest.ThemeTest

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

[simonhong]

Kindly ping.. I think this PR would not be affected by chromium's in-progress platform theme type checking because this only depends on our active theme type.

[petemill]

This works great. The one issue I have is that if the user changes the devtools Theme setting, there is no option to clear it and start 'tracking' the UI theme. However, the primary goal of this is to make it so that initially when the user changes the UI theme the devtools theme is changed. It's usually not a common user task to switch between themes, and if the user has changed devtools theme specifically, then they know where that setting is, so can change it there too. Just something to keep in mind when we consider the OS-level vs Browser-level (and now devtools-level) choice for themes.

It's probably just simplest for the user and for the code to remove the setting in devtools entirely, and have it track the single browser-level setting. When we clean up the settings for the OS-level choice, perhaps we can consider that.