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This patch adds impersonation of the anonymous token around calls
to the CloseClipboard system call. On Windows 8+ the win32k driver
captures the access token of the caller and makes it available to
other users on the desktop through the system call
GetClipboardAccessToken. This introduces a risk of privilege
escalation in sandboxed processes. By performing the impersonation
then whenever Chrome writes data to the clipboard only the anonymous
token is available.

BUG=440693

Review URL: https://codereview.chromium.org/792413003

Cr-Commit-Position: refs/heads/master@{#308372}