Don't pass -fno-cxa-atexit to clang-cl when enabling code coverage

On Windows, clang always uses regular atexit, so this wouldn't do
anything even if clang-cl did understand the flag.

R=mmoroz@chromium.org
BUG=846918

Change-Id: I59523e818488b5de3331a7851ae51a381a3e5c17
Reviewed-on: https://chromium-review.googlesource.com/1081202
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563460}
diff --git a/build/config/coverage/BUILD.gn b/build/config/coverage/BUILD.gn
index 6a6ec88..e407cb4 100644
--- a/build/config/coverage/BUILD.gn
+++ b/build/config/coverage/BUILD.gn
@@ -9,7 +9,6 @@
     cflags = [
       "-fprofile-instr-generate",
       "-fcoverage-mapping",
-      "-fno-use-cxa-atexit",
 
       # Following experimental flags removes unused header functions from the
       # coverage mapping data embedded in the test binaries, and the reduction
@@ -19,5 +18,9 @@
       "-limited-coverage-experimental=true",
     ]
     ldflags = [ "-fprofile-instr-generate" ]
+
+    if (!is_win) {
+      cflags += [ "-fno-cxa-atexit" ]
+    }
   }
 }