[Extensions] (Mostly) remove GetAssociatedWebContentsDeprecated()

All callers except the fileManagerPrivate API have been updated to not
rely on GetAssociatedWebContentsDeprecated(). The fileManagerPrivate
issue is being tracked separately. Remove the function
GetAssociatedWebContentsDeprecated() and move it to be directly in the
fileManagerPrivate API to avoid having usage spread.

Bug: 461394, 844654
Change-Id: I69adc7f3534d4908497bf73d3fcab0f7443450e7
Reviewed-on: https://chromium-review.googlesource.com/1066733
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560171}

• chrome/browser/chromeos/extensions/file_manager/private_api_dialog.cc[diff]
• chrome/browser/extensions/chrome_extension_function.cc[diff]
• chrome/browser/extensions/chrome_extension_function.h[diff]
• chrome/browser/extensions/chrome_extension_function_details.cc[diff]
• chrome/browser/extensions/chrome_extension_function_details.h[diff]
• extensions/browser/extension_function.cc[diff]
• extensions/browser/extension_function.h[diff]