Disable code coverage config for nonsfi_libc_free_nexe target.

The same reason as for sanitizers that are already disabled, as the target
doesn't use libc which is needed for compiler-rt instrumentations.

Bug: 789981, 759794
Change-Id: I3a997bd017154ba1fc41b3b39ff74af3627637a5
Reviewed-on: https://chromium-review.googlesource.com/1034284Reviewed-by: Mark Seaborn <mseaborn@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554757}

chrome/test/data/nacl/BUILD.gn

@@ -607,8 +607,9 @@ if (is_linux && is_clang) {
     defines = [ "NACL_LINUX=1" ]
     include_dirs = [ "../../../.." ]
 
-    # Do not use any sanitizers tools, which require a few symbols.
+    # Do not use any sanitizers or coverage tools, which require a few symbols.
     configs -= [ "//build/config/sanitizers:default_sanitizer_flags" ]
+    configs -= [ "//build/config/coverage:default_coverage" ]
     configs += [ ":nonsfi_libc_free_nexe_config" ]
   }