hterm: move from non-standard -webkit-transition CSS to standard transition
hterm: move from non-standard -webkit-transition CSS to standard transition

The standard name has been supported for a long time at this point.

Bug: https://crbug.com/609303
Change-Id: I74c6b51d9aaf53b1e15a19544b74f7a4c5d1fa97
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2563926
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm