Switching libfuzzer to trace-pc-guard instrumentation.

This reverts commit 23d6de2177b812285946bce6f459815799d825c5.

BUG=651540,687570

Review-Url: https://codereview.chromium.org/2721593003
Cr-Original-Commit-Position: refs/heads/master@{#453396}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fd938bac4105603ff6a997d5de3cabe39d8869c0

config/sanitizers/sanitizers.gni

@@ -140,7 +140,7 @@ declare_args() {
   asan_globals = !is_mac
 }
 
-if (use_afl && sanitizer_coverage_flags == "") {
+if ((use_afl || use_libfuzzer) && sanitizer_coverage_flags == "") {
   sanitizer_coverage_flags = "trace-pc-guard"
 } else if (use_sanitizer_coverage && sanitizer_coverage_flags == "") {
   sanitizer_coverage_flags = "edge,indirect-calls,8bit-counters"