Make Flash plan for the first release

[bbondy]

Chrome's Flash deprecation policy is here:
https://www.chromium.org/flash-roadmap

As of C68 we'll have:

Non-Persisted HTML5 by Default (Target: Chrome 68 - July 2018)
Summary
Sites using Flash will require explicit permission to run, every time the user restarts the browser.
Rationale
Require affirmative user choice to run Flash Player content, without that choice persisting across multiple sessions.

For the initial version that is OK.

We can diverge from the Disable by default (Chrome 76+ - July 2019) and Remove Flash completely ( Dec 2020) in separate issues.

[bbondy]

CC @diracdeltas in case there are objections. See above #30 (comment)

[bbondy]

Talked to @diracdeltas and she'd like to match this milestone:
https://www.chromium.org/flash-roadmap#TOC-Flash-Disabled-by-Default-Target:-Chrome-76---July-2019-

So re-opening.

[srirambv]

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	Linux

• Verified visiting http://homestarrunner.com/ showed Plugin blocked message on URL bar
• Verified clicking Run flash this time reloads the page
• Verified Enable Plugin is shown in right click context menu after page reloads, clicking it loads the flash content

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	Windows

Verification 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

• Verified visiting http://homestarrunner.com/ showed Plugin blocked message on URL bar
• Verified clicking Run flash this time reloads the page
• Verified Enable Plugin is shown in right click context menu after page reloads, clicking it shows prompt, clicking Allow loads the flash content
• Verified clicking on the 'Click to enable Adobe Flash Player', clicking it shows prompt, clicking Allow loads the flash content