gn: Cleanup - Sort GYP sources list alphabetical.

This was generated using the following command line:

$ tools/sort_sources.py tools/gn/gn.gyp

BUG=None
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/977603002

Cr-Commit-Position: refs/heads/master@{#319137}

tools/gn/gn.gyp

@@ -30,8 +30,8 @@
         'command_check.cc',
         'command_clean.cc',
         'command_desc.cc',
-        'command_gen.cc',
         'command_format.cc',
+        'command_gen.cc',
         'command_help.cc',
         'command_ls.cc',
         'command_refs.cc',
@@ -53,13 +53,10 @@
         'err.h',
         'escape.cc',
         'escape.h',
-        'exec_process.h',
         'exec_process.cc',
+        'exec_process.h',
         'filesystem_utils.cc',
         'filesystem_utils.h',
-        'functions_target.cc',
-        'functions.cc',
-        'functions.h',
         'function_exec_script.cc',
         'function_foreach.cc',
         'function_get_label_info.cc',
@@ -73,6 +70,9 @@
         'function_template.cc',
         'function_toolchain.cc',
         'function_write_file.cc',
+        'functions.cc',
+        'functions.h',
+        'functions_target.cc',
         'group_target_generator.cc',
         'group_target_generator.h',
         'header_checker.cc',
@@ -106,12 +106,12 @@
         'ninja_copy_target_writer.h',
         'ninja_group_target_writer.cc',
         'ninja_group_target_writer.h',
-        'ninja_utils.cc',
-        'ninja_utils.h',
         'ninja_target_writer.cc',
         'ninja_target_writer.h',
         'ninja_toolchain_writer.cc',
         'ninja_toolchain_writer.h',
+        'ninja_utils.cc',
+        'ninja_utils.h',
         'ninja_writer.cc',
         'ninja_writer.h',
         'operators.cc',
@@ -170,9 +170,9 @@
         'tool.h',
         'toolchain.cc',
         'toolchain.h',
-        'unique_vector.h',
         'trace.cc',
         'trace.h',
+        'unique_vector.h',
         'value.cc',
         'value.h',
         'value_extractors.cc',