googletest/README.libaom: update 'Revision'

Use the commit hash rather than the tag. Fixes a Chromium automation
error:

Revision is not a valid hexadecimal revision.
Revisions must be hexadecimal strings with a length of 7 to 40 characters.

Bug: 482449277
Change-Id: I616dc4334c15a799f2a5f15fa4e4b334155c1ce3

third_party/googletest/README.libaom

diff --git a/third_party/googletest/README.libaom b/third_party/googletest/README.libaom
index 624c939..2b898a2 100644
--- a/third_party/googletest/README.libaom
+++ b/third_party/googletest/README.libaom
@@ -3,7 +3,7 @@
 URL: https://github.com/google/googletest
 Version: 1.12.1
 Update Mechanism: Manual
-Revision: release-1.12.1
+Revision: 58d77fa8070e8cec2dc1ed015d66b454c8d78850
 License: BSD-3-Clause
 License File: LICENSE
 Shipped in Chromium: no