This affects multiple tests:
ProfilingJsonExporterTest.MemoryMaps
ProfilingJsonExporterTest.Metadata
ProfilingJsonExporterTest.DumpsHeader
ProfilingJsonExporterTest.Simple
ProfilingJsonExporterTest.SimpleWithFilteredAllocations
ProfilingJsonExporterTest.Context

First failing build: https://build.chromium.org/p/chromium.clang/builders/ToTLinuxMSan/builds/602

Example log: https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.clang%2FToTLinuxMSan%2F602%2F%2B%2Frecipes%2Fsteps%2Funit_tests%2F0%2Flogs%2FProfilingJsonExporterTest.MemoryMaps%2F0

Excerpt:
[ RUN      ] ProfilingJsonExporterTest.MemoryMaps
==2766==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x29059011 in ProcessNameFromProcessType chrome/profiling/json_exporter.cc:94:3
    #1 0x29059011 in WriteProcessName chrome/profiling/json_exporter.cc:128:0
    #2 0x29059011 in profiling::ExportAllocationEventSetToJSON(int, profiling::ExportParams const&, std::__1::unique_ptr<base::DictionaryValue, std::__1::default_delete<base::DictionaryValue> >, std::__1::basic_ostream<char, std::__1::char_traits<char> >&) chrome/profiling/json_exporter.cc:444:0
    #3 0x9ce1a55 in profiling::ProfilingJsonExporterTest_MemoryMaps_Test::TestBody() chrome/profiling/json_exporter_unittest.cc:548:3
    #4 0xa020525 in testing::Test::Run() third_party/googletest/src/googletest/src/gtest-internal-inl.h:0:10
    #5 0xa023ee4 in testing::TestInfo::Run() third_party/googletest/src/googletest/src/gtest.cc:2654:11
    #6 0xa0258b9 in testing::TestCase::Run() third_party/googletest/src/googletest/src/gtest.cc:2772:28
    #7 0xa046af4 in testing::internal::UnitTestImpl::RunAllTests() third_party/googletest/src/googletest/src/gtest.cc:4677:43
    #8 0xa045949 in testing::UnitTest::Run() third_party/googletest/src/googletest/src/gtest.cc:0:0
    #9 0x11ca0893 in RUN_ALL_TESTS third_party/googletest/src/googletest/include/gtest/gtest.h:2237:46
    #10 0x11ca0893 in base::TestSuite::Run() base/test/test_suite.cc:270:0
    #11 0x11caaa39 in Run base/callback.h:92:12
    #12 0x11caaa39 in base::(anonymous namespace)::LaunchUnitTestsInternal(base::RepeatingCallback<int ()> const&, unsigned long, int, bool, base::RepeatingCallback<void ()> const&) base/test/launcher/unit_test_launcher.cc:216:0
    #13 0x11caa25f in base::LaunchUnitTests(int, char**, base::RepeatingCallback<int ()> const&) base/test/launcher/unit_test_launcher.cc:475:10
    #14 0x11c7f4c5 in main chrome/test/base/run_all_unittests.cc:30:10
    #15 0x7f27d4175f44 in __libc_start_main /build/eglibc-SvCtMH/eglibc-2.19/csu/libc-start.c:287:0
    #16 0x21f5028 in _start ??:0:0
  Uninitialized value was created by an allocation of 'params' in the stack frame of function '_ZN9profiling41ProfilingJsonExporterTest_MemoryMaps_Test8TestBodyEv'
    #0 0x9ce05d0 in profiling::ProfilingJsonExporterTest_MemoryMaps_Test::TestBody() chrome/profiling/json_exporter_unittest.cc:536:0
SUMMARY: MemorySanitizer: use-of-uninitialized-value (/b/s/w/ir/out/Release/unit_tests+0x29059011)
Exiting