mesa: Disable support for loading libtxc_dxtn.so if msan is enabled.

third_party/mesa/BUILD.gn

Index: third_party/mesa/BUILD.gn
diff --git a/third_party/mesa/BUILD.gn b/third_party/mesa/BUILD.gn
index d4b1f99230975ece4d66919fcc78439f41c009ef..03c52a397390c304905f2bc8618aef6a0ec39bf8 100644
--- a/third_party/mesa/BUILD.gn
+++ b/third_party/mesa/BUILD.gn
@@ -59,7 +59,6 @@ config("mesa_internal_config") {
     "USE_XCB",
     "GLX_INDIRECT_RENDERING",
     "GLX_DIRECT_RENDERING",
-    "USE_EXTERNAL_DXTN_LIB=1",
     "IN_DRI_DRIVER",
     "HAVE_ALIAS",
     "HAVE_MINCORE",
@@ -114,6 +113,14 @@ config("mesa_internal_config") {
       cflags += [ "-fPIC" ]
     }
   }
+
+  if (is_msan) {
+    # Disable code in mesa that loads the libtxc_dxtn.so library if msan is
+    # enabled, as the library will be loaded from a system directory, which
+    # may cause spurious msan reports because msan expects most libraries in
+    # the process to be compiled with msan.
+    defines += [ "USE_EXTERNAL_DXTN_LIB=0" ]
+  }
 }
 
 # mesa_internal_config is prepended to the config lists.  Flags that