Add barhopper to third_party/.gitignore.

This CL adds //third_party/barhopper to .gitignore. This repo is
imported from src-internal and is used for implementing Chrome's
shape-detection API's in ChromeOS. DEPS entry previously added at
https://crrev.com/c/2416746.

Bug: 1093185
Change-Id: I714456bfa8b484be58381c4cb34fe6c1a5e6d943
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422821Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809462}

third_party/.gitignore

@@ -39,6 +39,7 @@
 /arcore-android-sdk/src/
 /arcore-android-sdk/test-apks/arcore/*.apk
 /asan
+/barhopper
 /bazel/desugar/*.jar
 /binutils
 /bison