histograms.xml: Corrections for ChromiumAndroidLinker.*

Followup to http://crrev.com/843584, where the wrong histogram was taken
to rephrase the moment when it is recorded. Correcting the description
for:
  ChromiumAndroidLinker.RelroAvailableImmediately,
  ChromiumAndroidLinker.RelroSharingStatus

Bug: 1154224
Change-Id: Ic19053752a82eb7ff1f12e8777510d66f01467e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630705
Commit-Queue: Egor Pasko <pasko@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Egor Pasko <pasko@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843800}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -2295,10 +2295,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   <owner>lizeb@chromium.org</owner>
   <owner>pasko@chromium.org</owner>
   <summary>
-    Recorded when the native library is loaded by the Chromium linker, and an
-    attempt to spawn a shared RELRO region is made. Indicates whether the shared
-    RELRO was available synchronously right after the native library was loaded.
-    ModernLinker only.
+    Recorded when RELRO is replaced with a shared memory region, in all
+    processes consuming RELRO. Indicates whether the shared RELRO was available
+    synchronously right after the native library was loaded. ModernLinker only.
   </summary>
 </histogram>
 
@@ -2323,9 +2322,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   <owner>lizeb@chromium.org</owner>
   <owner>pasko@chromium.org</owner>
   <summary>
-    In each process that has successfully loaded the native library indicates
-    whether using shared RELRO was attempted and what the result was. Currently
-    it is only reported in ModernLinker.
+    Recorded when RELRO is replaced with a shared memory region, in all
+    processes consuming RELRO. Indicates whether the replacement was a success.
+    ModernLinker only.
   </summary>
 </histogram>