Remove third_party/perfetto prefix from the import

proto_library build rule for extension already has import_dirs pointing to //third_party/perfetto, which is needed for imports from track_event.proto to work. This makes it possible to use paths without third_party/perfetto, making verbatim copies of extension into Perfetto repository possible.

Bug: 161696674
Change-Id: I6f77f1e8670599b48da464ddf57b71597941de36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462129
Commit-Queue: Andrew Shulaev <ddrone@google.com>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Auto-Submit: Andrew Shulaev <ddrone@google.com>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815598}

base/tracing/protos/chrome_track_event.proto

@@ -4,7 +4,7 @@
 
 syntax = "proto2";
 
-import "third_party/perfetto/protos/perfetto/trace/track_event/track_event.proto";
+import "protos/perfetto/trace/track_event/track_event.proto";
 
 package perfetto.protos;