add a feature flag for enabling translation

[rebron]

Test Plan

• Outlined under Disable in-page translation in release channel brave-core#2542

Description

Add a feature flag for enabling translation service. Translation prompt/icon should not show in the url bar and translation context menu "Translate to 'language'" should not appear.

Feature flag for translation should be enabled for beta/dev/nightly channel.
Feature flag for translation should be disabled for release channel.

Brave version (brave://version info)

Feature flag should be disabled for release channel starting with 65.x.

[kjozwiak]

Verification PASSED on macOS 10.14.5 x64 using the following build:

Brave	0.65.115 Chromium: 75.0.3770.38 (Official Build) (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Mac OS X

• went through the test plan outlined under Disable in-page translation in release channel brave-core#2542

• ensured that the the translate model and Offer to translate pages that aren't in a language you read are still available under the other channels

• 0.65.115 Chromium: 75.0.3770.38 Example:

• 0.68.5 Chromium: 75.0.3770.38 & 0.66.80 Chromium: 75.0.3770.38 Example:

Verification passed on

Brave	0.65.116 Chromium: 75.0.3770.38 (Official Build) (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified the test plan outlined under Disable in-page translation in release channel brave-core#2542

[srirambv]

Verification passed on

Brave	0.65.116 Chromium: 75.0.3770.38 (Official Build) (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Linux

• Verified test plan and no option to Offer to translate shown in languages
• Found issue on all channel so logged Offer to translate button is missing on Linux #4664 User misunderstanding on the issue.