Use a single sequenced task runner for HTTPSEverywehre && level_db init fix

[bbondy]

Use a single sequenced task runner for HTTPSEverywehre

Otherwise we're creating lots of different threads and accessing things for the DB on different possibly overlapping threads.

Fix brave/brave-browser#896

I think this also fixes:

• Fix Brave web store page is causing webview crash brave-browser#707
• Fix Browser webview crash when navigating back on any page brave-browser#708
• Fix Browser crashes when loading a site via toptile in new tab brave-browser#739
• Fix Web view crash for nytimes  brave-browser#797
• Fix Neowin.net webview crashes on first launch. brave-browser#761
• Fix Page refresh is causing the Browser Crash brave-browser#762
• Fix Browser crashes for cnn.com website brave-browser#796
• Fix Web view crash for nytimes  brave-browser#797
• Fix Web view or Browser crashes for few non secure sites in first launch brave-browser#798
• Fix Browser crash when disconnecting slack call brave-browser#861

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Needed or QA/No-QA-Needed) to include the closed issue in milestone

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[simonhong]

LGTM