Bump minimum Chrome version to 66

[rekmarks]

This PR updates our minimum supported Chrome version from 63 to 66, so that we may use the AbortController browser API without polyfilling it.

Our minimum Firefox version supports the AbortController, but our current minimum Chrome version (63, released in December 2017) does not. Chrome shipped the AbortController in version 66, in April 2018. We have determined that an extremely small number of users are on Chrome 63 < 66, and that this change is therefore acceptable.

[Gudahtt]

The minimum browser versions are also in the manifest - it should be updated there as well.

[Gudahtt]

LGTM!

[adonesky1]

LGTM!

[metamaskbot]

Builds ready [200865d]

• builds: chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • phishing-detect: phishing-detect
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5

Page Load Metrics (462 ± 44 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	378	655	458	87	42
		domContentLoaded	362	645	440	91	44
		load	380	692	462	92	44
		domInteractive	362	645	440	91	44