Updated generate_milestone_reports.py with M78 version

Change-Id: Ifac41db781e6e0e9d8e131044fe1ec307ed3f3d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801987
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698896}

tools/binary_size/generate_milestone_reports.py

@@ -68,8 +68,9 @@ _DESIRED_VERSIONS = [
     '73.0.3683.75',
     '74.0.3729.112',
     '75.0.3770.143',
-    '76.0.3809.62',
-    '77.0.3861.0',  # Canary
+    '76.0.3809.132',
+    '77.0.3865.73',  # Beta
+    '78.0.3904.8',  # Also Beta
 ]