Remove v1 of Startup.FirstWebContents.* stats deprecated in M48.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index a3a3713ad18a761f4bc227c32c3be30c0830329e..fb03f5b434d89dc3f23e4a49667ee0ec761b4597 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -45036,8 +45036,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>erikchen@chromium.org</owner>
   <owner>gab@chromium.org</owner>
   <summary>
-    [Desktop] DEPRECATED. Measure the elapsed time from process launch to the
-    first main frame load of the first web contents. Deprecated in favor of
+    [Desktop] DEPRECATED (but kept as a known basis until M48 is phased out).
+    Measure the elapsed time from process launch to the first main frame load of
+    the first web contents. Deprecated in favor of
     Startup.FirstWebContents.MainFrameLoad2 which now avoids counting ill-cases
     (ref. FirstWebContentsProfiler::FinishReason).
   </summary>
@@ -45077,8 +45078,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>erikchen@chromium.org</owner>
   <owner>gab@chromium.org</owner>
   <summary>
-    [Desktop] DEPRECATED. Measure the elapsed time from process launch to the
-    first non- empty paint of the first web contents. Deprecated in favor of
+    [Desktop] DEPRECATED (but kept as a known basis until M48 is phased out).
+    Measure the elapsed time from process launch to the first non- empty paint
+    of the first web contents. Deprecated in favor of
     Startup.FirstWebContents.NonEmptyPaint2 which now avoids counting ill-cases
     (ref. FirstWebContentsProfiler::FinishReason).
   </summary>