[WebKit] Set C++ STD library to 14.

The build breaks with library version set to 17

Test: Successfully built pre-built.
Change-Id: I5c46aa4058f4df21754fc4dd5f122e6b3957bede
diff --git a/BUILD.gn b/BUILD.gn
index dc3d321..8c37fb8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -72,6 +72,7 @@
 config("flags") {
   cflags = [
     "-fno-sanitize=safe-stack",
+
     # TODO(phosek): This is consistent with how we were building WebKit before
     # the default flags changed. However, it is likely that using
     # stack-protector-strong is just fine. We should drop -fno-stack-protector
@@ -100,6 +101,8 @@
     "-Wundef",
     "-Wwrite-strings",
   ]
+
+  cflags_cc = [ "-std=c++14" ]
 }
 
 source_set("WebKit_source_set") {
@@ -116,9 +119,7 @@
     ":features",
     ":flags",
   ]
-  configs -= [
-    "//build/config:symbol_visibility_hidden",
-  ]
+  configs -= [ "//build/config:symbol_visibility_hidden" ]
 
   defines = [ "BUILDING_WebKit" ]