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We want to improve the installation time for force installed
extensions. For this, we need to first gather the statistics regarding
which installation stages are time consuming.

This CL adds the UMA statistics for the following installation stages:
1) Unpacking - Time taken to complete the unpacking of the extension
archive.
2) Checking - Time taken to perform the final checks to confirm that the
extension can be installed.
3) Finalizing - Time taken to install the extension.

Following histograms are added:
Extensions.ForceInstalledTime.UnpackingStartTo.CheckingStart
Extensions.ForceInstalledTime.CheckingStartTo.FinalizingStart
Extensions.ForceInstalledTime.FinalizingStartTo.CRXInstallComplete

Bug: 1102369
Change-Id: I46ed872a2954edba51ebdaeab8ab0ca9e1f4703b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351996Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Swapnil Gupta <swapnilgupta@google.com>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Cr-Commit-Position: refs/heads/master@{#799034}