Update build_ffmpeg.py ChromeOs instructions.

wez@ was unable to build using the listed instructions. These roll
document has different instructions which are less prone to rot, so
simply link to the doc.

Change-Id: I25be29978aec7a07c5e4239baf794b66eb4b5498
Reviewed-on: https://chromium-review.googlesource.com/716672
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
diff --git a/chromium/scripts/build_ffmpeg.py b/chromium/scripts/build_ffmpeg.py
index 7771194..8ba0318 100755
--- a/chromium/scripts/build_ffmpeg.py
+++ b/chromium/scripts/build_ffmpeg.py
@@ -48,15 +48,15 @@
     Script can run on a normal Ubuntu box.
 
   linux mipsel:
-    Script must be run inside of ChromeOS SimpleChrome setup:
-        cros chrome-sdk --board=mipsel-o32-generic --use-external-config
+    Script must be run inside of ChromeOS checkout. Follow instructions in this doc:
+        https://docs.google.com/document/d/14bqZ9NISsyEO3948wehhJ7wc9deTIz-yHUhF1MQp7Po/edit?pref=2&pli=1#bookmark=id.9xsx8g59oaay
 
   linux mips64el:
     Script can run on a normal Ubuntu with mips64el cross-toolchain in $PATH.
 
   linux arm/arm-neon:
-    Script must be run inside of ChromeOS SimpleChrome setup:
-        cros chrome-sdk --board=arm-generic
+    Script must be run inside of ChromeOS checkout. Follow instructions in this doc:
+        https://docs.google.com/document/d/14bqZ9NISsyEO3948wehhJ7wc9deTIz-yHUhF1MQp7Po/edit?pref=2&pli=1#bookmark=id.9xsx8g59oaay
 
   linux arm64:
     Script can run on a normal Ubuntu with AArch64 cross-toolchain in $PATH.