Incrementing VERSION to 91.0.4472.6

Change-Id: Ic64ac4e57fba85e60d33af9e833994b9f79fd969
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2820325
Bot-Commit: Chrome Release Bot (LUCI) <chrome-official-brancher@chops-service-accounts.iam.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/4472@{#13}
Cr-Branched-From: 3d60439-refs/heads/master@{#870763}

chrome/VERSION

@@ -1,4 +1,4 @@
 MAJOR=91
 MINOR=0
 BUILD=4472
-PATCH=5
+PATCH=6