(It's possible that https://codereview.chromium.org/2803083002 will fix this, but filing this to make sure we don't lose track of it.)

First failing build: https://build.chromium.org/p/chromium.fyi/builders/ClangToTLinuxUBSanVptr%20tester/builds/1237

Logs: https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Fchromium.fyi%2FClangToTLinuxUBSanVptr_tester%2F1237%2F%2B%2Frecipes%2Fsteps%2Fangle_unittests%2F0%2Fstdout

Example:
Note: Google Test filter = ValidationESTest.DrawElementsWithMaxIndexGivesError
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from ValidationESTest
[ RUN      ] ValidationESTest.DrawElementsWithMaxIndexGivesError
../../third_party/angle/src/libANGLE/Framebuffer.cpp:1361:24: runtime error: downcast of address 0x7ffc30b9a8b8 which does not point to an object of type 'const gl::Context'
0x7ffc30b9a8b8: note: object is of type 'testing::NiceMock<(anonymous namespace)::MockValidationContext>'
 00 00 00 00  98 08 05 01 00 00 00 00  03 00 00 00 00 00 00 00  b8 a8 b9 30 fc 7f 00 00  a8 aa b9 30
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'testing::NiceMock<(anonymous namespace)::MockValidationContext>'
    #0 0xbcb77e  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xbcb77e)
    #1 0xbfeb03  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xbfeb03)
    #2 0xc02893  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xc02893)
    #3 0x75259e  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0x75259e)
    #4 0xb87360  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb87360)
    #5 0xb892ed  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb892ed)
    #6 0xb8abf2  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb8abf2)
    #7 0xb9c8e7  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb9c8e7)
    #8 0xb9b8da  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb9b8da)
    #9 0xb46861  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb46861)
    #10 0xa79c2b  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xa79c2b)
    #11 0xb49203  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb49203)
    #12 0xb497e0  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xb497e0)
    #13 0xa79b41  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0xa79b41)
    #14 0x7feab51e8f44  (/lib/x86_64-linux-gnu/libc.so.6+0x21f44)
    #15 0x4b4805  (/b/s/w/irVwIo5T/out/Release/angle_unittests+0x4b4805)