Add CHROMECAST_EXPORT to SetHdmiContentType()

Also, verified that such function is visible.

Bug: b/131411395
Test: With internal CLs, build on one device and cast a youtube page.
Change-Id: I55ae72fa2ae71ab6495e7b32c16150a678086773
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721555
Commit-Queue: Guohui Deng <guohuideng@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682377}

chromecast/public/avsettings.h

@@ -233,7 +233,7 @@ class AvSettings {
   // supported by Hdmi sink, in which case implementation shall return false
   // without actually setting the content type.
   // This function should only be implemented on HDMI platforms.
-  static bool SetHdmiContentType(HdmiContentType content_type)
+  static CHROMECAST_EXPORT bool SetHdmiContentType(HdmiContentType content_type)
       __attribute__((weak));
 
   // Gets the HDMI latency in microseconds.