Add rtc_use_memcheck flag, update MB and GN to handle it, and add gni files listing the runtime deps

webrtc/build/mb_config.pyl

Index: webrtc/build/mb_config.pyl
diff --git a/webrtc/build/mb_config.pyl b/webrtc/build/mb_config.pyl
index 2a86e6218db58740fc63f7cb4a6dad1d6bebb60b..fceca6034e15977a17cba5033113d4c7ca63749e 100644
--- a/webrtc/build/mb_config.pyl
+++ b/webrtc/build/mb_config.pyl
@@ -40,7 +40,7 @@
       'Linux64 Debug': 'debug_bot_x64',
       'Linux64 Release': 'release_bot_x64',
       'Linux Asan': 'asan_lsan_clang_release_bot_x64',
-      'Linux Memcheck': 'release_bot_x64',
+      'Linux Memcheck': 'memcheck_release_bot_x64',
       'Linux MSan': 'msan_clang_release_bot_x64',
       'Linux Tsan v2': 'tsan_clang_release_bot_x64',
       'Linux UBSan': 'ubsan_clang_release_bot_x64',
@@ -231,6 +231,9 @@
     'perf_release_bot_x64': [
       'gn', 'openh264', 'release', 'static', 'goma', 'x64'
     ],
+    'memcheck_release_bot_x64': [
+      'memcheck', 'gn', 'openh264_release_bot', 'x64'
+    ],
 
     # Windows
     'win_clang_debug_bot_x86': [
@@ -368,6 +371,10 @@
       'gn_args': 'is_lsan=true',
     },
 
+    'memcheck': {
+      'gn_args': 'rtc_use_memcheck=true',
+    },
+
     'minimal_symbols': {
       'gn_args': 'symbol_level=1',
     },