Revert "Export angle::PlatformMethods."

This reverts commit 8b920b211d7363daef0d5fb8d2bd69f3253ab033.

Reason for revert: This had a syntax error and skipped the compile step, so went undetected.

BUG=chromium:778918

Original change's description:
> Export angle::PlatformMethods.
> 
> This should fix undefined function call sanitizer warnings.
> 
> BUG=chromium:778918
> 
> Change-Id: I718d58edd7b75a3fca1089edc4df7f3251d85153
> Reviewed-on: https://chromium-review.googlesource.com/767152
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>

TBR=jmadill@chromium.org,piman@chromium.org

Change-Id: I1825ce88bc82867a983bb08cd6732caff54f061c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:778918
Reviewed-on: https://chromium-review.googlesource.com/767128
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/include/platform/Platform.h b/include/platform/Platform.h
index 21bf644..a446283 100644
--- a/include/platform/Platform.h
+++ b/include/platform/Platform.h
@@ -264,7 +264,7 @@
 
 #define ANGLE_PLATFORM_METHOD_DEF(Name, CapsName) CapsName##Func Name = Default##CapsName;
 
-ANGLE_PLATFORM_EXPORT struct PlatformMethods
+struct PlatformMethods
 {
     // User data pointer for any implementation specific members. Put it at the start of the
     // platform structure so it doesn't become overwritten if one version of the platform