docs/gpu/vaapi.md: Link TODO to bug

Link TODO to bug

TBR=andrescj@chromium.org

Bug: 917091
Change-Id: I0489ca019a1ca300d198f2c4c1a6717f65ef78eb
Reviewed-on: https://chromium-review.googlesource.com/c/1387392Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618293}

docs/gpu/vaapi.md

@@ -210,7 +210,7 @@ To run Chromium using VaAPI two arguments are necessary:
 
 Also, the environment variable `MESA_GLSL_CACHE_DISABLE=false` should be set:
 
-**TODO(mcasas): Further clarify the env variable and the flags**
+**TODO(crbug.com/917091): Further clarify the env variable and the flags**
 
 ```shell
  MESA_GLSL_CACHE_DISABLE=false ./out/gn/chrome --ignore-gpu-blacklist --use-gl=egl