Rename source_prereqs to inputs in the GN Build file.

See more information in the gn-dev mailing list:
https://groups.google.com/a/chromium.org/d/msg/gn-dev/j-P5hlvZ13M/4CSw2WworlkJ

Also in
https://chromium.googlesource.com/chromium/src/+/61a6fca4bca5438bf4338184c29fded55e5f240e

BUG=374271
R=machenbach@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/917243002

Cr-Commit-Position: refs/heads/master@{#26658}

BUILD.gn

@@ -164,7 +164,7 @@ action("js2c") {
 
   # The script depends on this other script, this rule causes a rebuild if it
   # changes.
-  source_prereqs = [ "tools/jsmin.py" ]
+  inputs = [ "tools/jsmin.py" ]
 
   sources = [
     "src/runtime.js",
@@ -224,7 +224,7 @@ action("js2c_experimental") {
 
   # The script depends on this other script, this rule causes a rebuild if it
   # changes.
-  source_prereqs = [ "tools/jsmin.py" ]
+  inputs = [ "tools/jsmin.py" ]
 
   sources = [
     "src/macros.py",