AW UMA: persist session_id

This persists session_id, an integer which counts the current user
session with the app. This value is reported in UMA metrics.

Bug: 1004614
Test: startup webview shell multiple times, log session_id inside
Test: AwMetricsLogUploader::UploadLog, verify session_id increases
Change-Id: I3bb33e1b9443729d3d59c60bdd11af2e790c8512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842627Reviewed-by: Tao Bai <michaelbai@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703465}

android_webview/browser/aw_feature_list_creator.cc

@@ -51,6 +51,8 @@ const char* const kPersistentPrefsWhitelist[] = {
     // Random seed value for variation's entropy providers. Used to assign
     // experiment groups.
     metrics::prefs::kMetricsLowEntropySource,
+    // Logged directly in the ChromeUserMetricsExtension proto.
+    metrics::prefs::kMetricsSessionID,
     // Current and past country codes, to filter variations studies by country.
     variations::prefs::kVariationsCountry,
     variations::prefs::kVariationsPermanentConsistencyCountry,