Install pywin32 into depot_tools's python.

[mkarolin]

depot_tools is currently missing pywin32 on Windows which is needed
by build/toolchain/win/tool_wrapper.py.

Fixes: #7328

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).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection

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.

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.

[mkarolin]

CI passed: MacOS failed in the first job for an unrelated reason. MacOS succeeded in the second job.

[bsclifton]

++ LGTM! (tested and it works great)

[bsclifton]

NOTE: this only happens during init. Output looks like this:

Manually installing pywin32 python module
C:\brave-browser: python -m pip install pywin32
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
Collecting pywin32
  Downloading https://files.pythonhosted.org/packages/a2/e6/6b395ea8bc9500158adea646e28077adb7f3d9587c681fd5d03693a4b5dc/pywin32-227-cp27-cp27m-win_amd64.whl (7.4MB)
Installing collected packages: pywin32
Successfully installed pywin32-227