Prefix includes in html files generated by js_unit_test.py

This allows the html files generated under out/foo/gen/.... to be opened
in Chrome as a file:/// URL. It doesn't affect how the files are served
by the embedded test server (leading ../..'s are ignored).

Bug: 860355
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I6a7a8838c6158f9f5cdeb9fa8e0745c23d88f602
Reviewed-on: https://chromium-review.googlesource.com/c/1264095Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597016}

third_party/closure_compiler/js_unit_test.py

@@ -26,6 +26,8 @@ def main():
                       help='List of additional js files to load before others')
   parser.add_argument('-o', '--output',
                       help='Generated html output with flattened dependencies')
+  parser.add_argument('-I', '--include_prefix', default='',
+                      help='Prefix added to src= paths to find .js files')
   args = parser.parse_args()
 
   uniquedeps = Flatten([args.input])
@@ -46,9 +48,9 @@ window.addEventListener('error', function(e) {
 
 """)
     for file in args.mocks:
-      out.write('<script src="%s"></script>\n' % (file))
+      out.write('<script src="%s%s"></script>\n' % (args.include_prefix, file))
     for file in uniquedeps:
-      out.write('<script src="%s"></script>\n' % (file))
+      out.write('<script src="%s%s"></script>\n' % (args.include_prefix, file))
     out.write('</body>\n</html>\n')
 
 if __name__ == '__main__':

third_party/closure_compiler/js_unit_tests.gni

@@ -61,6 +61,14 @@ template("js_unit_tests") {
     args = [ "--output" ] + rebase_path(outputs, root_build_dir)
     args += [ "--input" ] + [ "{{source}}" ]
 
+    # js_unit_test.py needs to walk .js_library dependency files relative to the
+    # gen_dir, but we'd like to reference the corresponding .js source files
+    # in the source dir, not the gen dir. Pass a prefix to remap --input args.
+    args += [
+      "--include_prefix",
+      rebase_path("//", "{{root_gen_dir}}", "//"),
+    ]
+
     if (defined(mocks)) {
       args += [ "--mocks" ] + rebase_path(mocks, root_build_dir)
       data = mocks