Looks to be introduced by https://crrev.com/6e7fec1054456994b333257bcb1fb68c7212b31a

First build: https://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/6932

Sample output isn't very helpful:
UNINITIALIZED READ: reading 0x002af7dc-0x002af7dd 1 byte(s)
# 0 cc::`anonymous namespace'::LayerTreeSettingsTest_AllMembersChanged_Test::TestBody [cc\trees\layer_tree_settings_unittest.cc:77]
# 1 testing::internal::HandleExceptionsInMethodIfSupported<>                   [testing\gtest\src\gtest.cc:2458]
Note: @0:06:54.125 in thread 888
Note: instruction: cmp    0xffffffb0(%ebp) $0x00
Suppression (error hash=#72621E8C109BDCA7#):
For more info on using suppressions see http://dev.chromium.org/developers/how-tos/using-drmemory#TOC-Suppressing-error-reports-from-the-
{
UNINITIALIZED READ
name=<insert_a_suppression_name_here>
*!cc::`anonymous namespace'::LayerTreeSettingsTest_AllMembersChanged_Test::TestBody
*!testing::internal::HandleExceptionsInMethodIfSupported<>
}

Am guessing it is the now redundant proto field.