Use BLINK_PLATFORM_EXPORT instead of PLATFORM_EXPORT macro in media mojo

In Windows the media mojo code used PLATFORM_EXPORT, an internal
blink macro, in some generated code. This "worked" and it compiled
thanks to the blink internal platform_export.h header being
included through some other mistaken code.

As that other mistaken code stops including blink's platform_export.h this
will stop compiling as is.

Switching to the external BLINK_PLATFORM_EXPORT makes everything right.

Change-Id: If0b3de739e4953dfd211d1ffd6d7d44bc7f7d69e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1511411
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#639569}
diff --git a/media/mojo/interfaces/BUILD.gn b/media/mojo/interfaces/BUILD.gn
index ec8b4bb..c611503 100644
--- a/media/mojo/interfaces/BUILD.gn
+++ b/media/mojo/interfaces/BUILD.gn
@@ -72,7 +72,7 @@
   # classes. Enabling this for other builds would result in ODR violations.
   # TODO(crbug.com/921170): Remove this once the issue is resolved.
   if (is_win && is_component_build) {
-    export_class_attribute_blink = "PLATFORM_EXPORT"
+    export_class_attribute_blink = "BLINK_PLATFORM_EXPORT"
     export_define_blink = "BLINK_PLATFORM_IMPLEMENTATION=1"
     export_header_blink = "third_party/blink/public/platform/web_common.h"
   }