Add .gitignore entry for /third_party/google_benchmark/src

Change-Id: I41a0a70bbe417f0baaeccc0ff9514d1e20119eb3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/127880
Reviewed-by: Loko Kung <lokokung@google.com>
Auto-Submit: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
diff --git a/.gitignore b/.gitignore
index e73a73a..b048db9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,12 +11,12 @@
 /testing
 /third_party/abseil-cpp/
 /third_party/angle
-/third_party/benchmark
 /third_party/binutils
 /third_party/catapult
 /third_party/clang-format
 /third_party/cpplint
 /third_party/glfw
+/third_party/google_benchmark/src
 /third_party/googletest
 /third_party/gpuweb
 /third_party/jinja2