Rename WebFont.LayoutLatency histogram to WebFont.StyleRecalcToDownloadLatency

This change was suggested in https://codereview.chromium.org/23075006/, for clarity.

BUG=246492

Review URL: https://chromiumcodereview.appspot.com/22815024

git-svn-id: svn://svn.chromium.org/blink/trunk@156539 bbb929c8-8fbe-4397-9dbb-9b2b20218538