Add tools/code_coverage to .gitignore

This directory is pulled in via DEPS.

Change-Id: Ib0ec9fa153d07d718a78d5bc0e6fe9ca112610ae
Reviewed-on: https://pdfium-review.googlesource.com/33710
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/.gitignore b/.gitignore
index 0111ff0..e177305 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,7 @@
 /third_party/yasm/source/patched-yasm
 /third_party/zlib
 /tools/clang
+/tools/code_coverage
 /tools/gyp
 /tools/memory
 /v8