Let flatbuffer.gni supports to generate mutable header

This change adds a "mutable" option to flatbuffer template,
let it using "--gen-mutable" flag to compile flatbuffer with
non-const accessors for mutating FlatBuffers in-place.

Bug: 1149150
Test: Compile flatbuffer see if it generates mutable header
Change-Id: I42ce7dfcac5ee8496db56548391b205d0b056aa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534550
Commit-Queue: Fish Lin <linfish@google.com>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827709}

third_party/flatbuffers/flatbuffer.gni

@@ -25,6 +25,10 @@
 #       fail it will try to load relative to the directory of the schema file
 #       being parsed.
 #
+#   mutable (optional)
+#       Boolean to compile flatbuffers with the "--gen-mutable" argument, which
+#       generates non-const accessors for mutating FlatBuffers in-place.
+#
 #   deps (optional)
 #       Additional dependencies.
 #
@@ -90,6 +94,10 @@ template("flatbuffer") {
       }
     }
 
+    if (defined(invoker.mutable) && invoker.mutable) {
+      args += [ "--gen-mutable" ]
+    }
+
     args += [ "{{source}}" ]
 
     # The deps may have steps that have to run before running flatc.