Add Fuchsia OWNERS exceptions for content/*/*_fuchsia.cc.

Bug: 
Change-Id: I9b82e18e3803d5e80e9fba98c3e2be73013f215c
Reviewed-on: https://chromium-review.googlesource.com/630977
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497221}

content/OWNERS

@@ -22,4 +22,7 @@ per-file content_nacl_nonsfi.gyp=*
 per-file *.sb=set noparent
 per-file *.sb=rsesek@chromium.org
 
+# For Fuchsia-specific changes:
+per-file *_fuchsia*=file://build/fuchsia/OWNERS
+
 # COMPONENT: Internals>Core