Prompt user to close Brave before importing from a Brave profile

[garrettr]

Description

Follow-up for brave/brave-core#185.

The new BraveImporter needs a BraveProfileLock equivalent to the ChromeProfileLock implemented for ChromeImporter in brave/brave-core#101.

Brave browser-laptop is based on Muon/Chromium, so we should be able to reuse the same approach and probably also code from ChromeProfileLock for BraveProfileLock. I also want to refactor the lock dialog-related code in BraveExternalProcessImporterHost and brave::importer::ImportLockDialogView to minimize duplication of code and support long-term maintainability.

[btlechowski]

Verification passed on

Brave	0.55.5 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Windows 7

Verification Passed on

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Linux

Verified passed with

Brave	0.55.10 Chromium: 70.0.3538.22 (Official Build) beta(64-bit)
Revision	ac9418ba9c3bd7f6baaffa0b055dfe147e0f8364-refs/branch-heads/3538@{#468}
OS	Mac OS X

• used macOS 10.12.6

Went through verification using the following build under macOS 10.13.6 x64 - PASSED

Brave	0.55.10 Chromium: 70.0.3538.22 (Official Build) beta(64-bit)
Revision	ac9418ba9c3bd7f6baaffa0b055dfe147e0f8364-refs/branch-heads/3538@{#468}
OS	Mac OS X