Images over 32M pixels exceed IPC message limit size.  In ChromeContentUtilityClient::DecodeImage(), cut image dimensions in half to minimize quality loss and allow decoded image to be returned via IPC message.

BUG=366825
TEST=ChromeContentUtilityClientTest.DecodeImageSizeLimit; Set ChromeOS wallpaper using JPGs attached to bug

Review URL: https://codereview.chromium.org/482163002

Cr-Commit-Position: refs/heads/master@{#296407}