Expire Flash settings if it doesn't come for free in C68

[bbondy]

This should come for free in C68 here:
https://www.chromium.org/flash-roadmap#TOC-Non-Persisted-HTML5-by-Default-Target:-Chrome-68---July-2018-

But if not we should implement it.

[bbondy]

This came for free from the Chromium 70 upgrade.

[LaurenWags]

Verified passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) beta(64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Mac OS X

• Navigated to homestarrunner.com -> received notice in URL bar
  
• Clicked 'Run Flash this time'. Clicked 'Click to enable Adobe Flash Player'. Was prompted in URL bar to Allow or Block:
  
• Verified both Block and Allow function as expected.
• Navigated to chrome://settings/content/flash --> Verified setting toggle to 'Ask first' and navigating to flash site did not show Plugin blocked URL bar notice (had to click 'Click to enable Adobe Flash Player' and select block or allow from prompt)
• Verified flash settings were expired on restart per notice on chrome://settings/content/flash

Verification passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) (64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Windows 7

[srirambv]

Linux is blocked on #1562