[sample] use v8_monolith target to simplify hello world sample

New steps would be
- `tools/dev/v8gen.py x64.release.sample`
- `ninja -C out.gn/x64.release.sample v8_monolith`
- `g++ -I. -Iinclude samples/hello-world.cc -o hello_world
     lv8_monolith -Lout.gn/x64.release.sample/obj/ -pthread -std=c++0x`
- `./hello_world` (no more worrying about .bin files)

R=jkummerow@chromium.org, mths@chromium.org

Change-Id: I3bba03b4c3ed34daf242a570f420b90f94ec6de0
Reviewed-on: https://chromium-review.googlesource.com/1179663
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55574}

infra/mb/mb_config.pyl

@@ -54,6 +54,7 @@
       'x64.debug': 'default_debug_x64',
       'x64.optdebug': 'default_optdebug_x64',
       'x64.release': 'default_release_x64',
+      'x64.release.sample': 'release_x64_sample',
     },
     'client.dynamorio': {
       'linux-v8-dr': 'release_x64',
@@ -353,6 +354,8 @@
       'debug', 'x64', 'v8_enable_slow_dchecks'],
     'default_release_x64': [
       'release', 'x64'],
+    'release_x64_sample': [
+      'release', 'x64', 'sample'],
     'default_debug_x86': [
       'debug', 'x86', 'v8_enable_slow_dchecks', 'v8_full_debug'],
     'default_optdebug_x86': [
@@ -885,5 +888,9 @@
       'gn_args': 'target_cpu="x86"',
     },
 
+    'sample': {
+      'gn_args': 'v8_monolithic=true is_component_build=false '
+                 'v8_use_external_startup_data=false use_custom_libcxx=false',
+    },
   },
 }