Fix preprocessor failure in string concatenation.
Fix preprocessor failure in string concatenation.

BUG=angleproject:1138

Change-Id: I20f3b542884fbaa2b2cccd65ceb45e54174bccc2
Reviewed-on: https://chromium-review.googlesource.com/295831
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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