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* kPluginVmAppId -> kPluginVmShelfAppId: This id is also used by Plugin
  VM installer as the shelf id on the window. Rename to make it clearer.
* IsPluginVmWindow() -> IsPluginVmAppWindow(): This function only return
  true for the Plugin VM app window but not the installer window. Rename
  to make it clearer.

Bug: 1087726
Change-Id: Ib7cf899133ed970bd795c859ac740f5c15f77b23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247460Reviewed-by: Timothy Loh <timloh@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#779549}