Fixed crash when dragging entered into sidebar web panel

[simonhong]

fix brave/brave-browser#15505

Set dummy WebContentsDelegate to sidebar WebContents to fix crash.
Real delegate implementation will be done as a separate task.

Resolves

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[emerick]

LGTM

[simonhong]

Merged because only unrelated one test BraveAdsBrowserTest/BraveAdsUpgradeBrowserTest was failed.

[stephendonner]

Verified PASSED using

Brave	1.26.11 Chromium: 90.0.4430.93 (Official Build) nightly (x86_64)
Revision	4df112c29cfe9a2c69b14195c0275faed4e997a7-refs/branch-heads/4430@{#1348}
OS	macOS Version 11.3.1 (Build 20E241)

Was able to very easily reproduce this using earlier nightly builds; I followed the steps from brave/brave-browser#15505, and observed no crashes:

1. enabled sidebar feature via brave://flags and re-launched
2. loaded brave.com in a new tab
3. clicked sidebar's Brave Together panel
4. dragged brave.com tab's site info (security icon in omnibox) into sidebar web panel