Synchronize GN build configuration with Chrome

build_overrides/README.md

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+# Build overrides in GN
+
+This directory is used to allow different products to customize settings
+for repos that are DEPS'ed in or shared.
+
+For example: V8 could be built on its own (in a "standalone" configuration),
+and it could be built as part of Chromium. V8 might define a top-level
+target, //v8:d8 (a simple executable), that should only be built in the
+standalone configuration. To figure out whether or not it should be
+in a standalone configuration, v8 can create a file, build_overrides/v8.gni,
+that contains a variable, `build_standalone_d8 = true`.
+and import it (as import("//build_overrides/v8.gni") from its top-level
+BUILD.gn file.
+
+Chromium, on the other hand, might not need to build d8, and so it would
+create its own build_overrides/v8.gni file, and in it set 
+`build_standalone_d8 = false`. 
+
+The two files should define the same set of variables, but the values can
+vary as appropriate to the needs of the two different builds.