[Chromecast] Add speex and grpc to gitignore

Bug: None
Test: git status doesn't show speex and grpc
Change-Id: Ibcd02491cde56dc82a4eeffa9081f71b2dc61ea9
Reviewed-on: https://chromium-review.googlesource.com/1125282Reviewed-by: Bailey Forrest <bcf@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573934}

third_party/.gitignore

@@ -80,6 +80,7 @@
 /google_toolbox_for_mac/src
 /googlemac
 /googletest/src
+/grpc
 /gson/lib/
 /gvr-android-sdk/common_library.aar
 /gvr-android-sdk/test-libraries/controller_test_api.aar
@@ -189,6 +190,7 @@
 /skia
 /smhasher/src
 /snappy/src
+/speex
 /spirv-headers/src
 /SPIRV-Tools/src
 /spirv-tools-angle/src