[VizHitTesting] Extend expiry milestone for flag enable-viz-hit-test-surface-layer

VizHitTestSurfaceLayer will go 1% on stable in M77 (SR: Sep 10). In case
that it cannot be fully shipped in M77, it's better to have the flag
available in M78 (branch Sep 5) and M79 (branch Oct 17).

Bug: None
Change-Id: I0db79a07667ce18bafa66b6fd2f2e7a4b5f6aab6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762580
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688680}

chrome/browser/flag-metadata.json

@@ -1799,7 +1799,7 @@
   {
     "name": "enable-viz-hit-test-surface-layer",
     "owners": [ "yigu", "rjkroege", "kylechar" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 79
   },
   {
     "name": "enable-web-authentication-ble-support",