GN build fixes for Windows component build.
GN build fixes for Windows component build.

Fixes the defines for the platform heap target to match the GYP build. With this patch, blink compiles on Windows in component mode. The fixes also apply to Linux, but apparently the defines don't affect the linker in a fatal way.

Review URL: https://codereview.chromium.org/1066803002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@193247 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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