media: Disable widevine cdm for Linux arm64 builds.

Currently there are no widevine cdm libraries for arm64 builds.
Disable it to avoid build errors when building ARM64 Chrome
in Chrome OS.

Bug: 881022
Test: No more missing widevinecdm files errors.
Change-Id: I476415cef0c435cb50e113de4e576cb6d3006951
Reviewed-on: https://chromium-review.googlesource.com/1211974Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589983}

third_party/widevine/cdm/widevine.gni

@@ -14,7 +14,9 @@ enable_widevine_cdm_host_verification =
     (is_chrome_branded || enable_widevine) && enable_cdm_host_verification
 
 # Only bundle Widevine CDM in Google Chrome builds.
-should_bundle_widevine_cdm = is_chrome_branded && enable_library_cdms
+# TODO: Provide prebuilt libraries for ARM64.
+should_bundle_widevine_cdm =
+    is_chrome_branded && enable_library_cdms && target_cpu != "arm64"
 
 template("widevine_sign_file") {
   # For official builds, generate a signature file for |file| which will