[js2gtest] specify empty public header

This is to improve build throughput around js2gtest by removing
unnecessary build dependency.
Explanation of empty public is written in
https://gn.googlesource.com/gn/+/master/docs/reference.md#var_public

e.g. The number of build target for obj/chrome/test/browser_tests/profile_list_desktop_browsertest.o
reduced from 33553 to 5505 by this CL.

This dependency removal utilizes build parallelism well.

Bug: 937723
Change-Id: Ic92c80f2006578f9f43cc2e5d357809c414adfdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505512
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638171}

chrome/test/base/js2gtest.gni

@@ -118,6 +118,11 @@ template("js2gtest") {
                              "visibility",
                            ])
     sources = get_target_outputs(":$action_name")
+
+    # This empty public header is intentional to remove unnecessary build
+    # dependency.
+    public = []
+
     deps = [
       ":$action_name",