add "Platform.MountEncrypted.TimeToTakeTpmOwnership" for ChromeOS.

This would help developers understand how costly the TPM ownership
process is in real life.

For the corresponding ChromeOS change, see CL:2619438.

BUG=b:176463814
TEST=build ok.

Change-Id: I1bde763f3d776c16abc86ab4ddb4d340f37c3159
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2619359Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Leo Lai <cylai@google.com>
Cr-Commit-Position: refs/heads/master@{#843107}

tools/metrics/histograms/histograms_xml/platform/histograms.xml

@@ -725,6 +725,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Platform.MountEncrypted.TimeToTakeTpmOwnership" units="ms"
+    expires_after="2021-12-31">
+  <owner>apronin@chromium.org</owner>
+  <owner>mnissler@chromium.org</owner>
+  <owner>cylai@google.com</owner>
+  <owner>cros-hwsec+uma@chromium.org</owner>
+  <summary>
+    The amount of time for Chrome OS mount-encrypted to take TPM ownership.
+  </summary>
+</histogram>
+
 <histogram name="Platform.PageFaultsLong" units="page faults/second"
     expires_after="2021-07-02">
   <owner>sonnyrao@chromium.org</owner>