Remove EncodingStreams runtime flag

TextEncoderStream and TextDecoderStream have shipped. The flag is no
longer needed. Remove it.

BUG=898158

Change-Id: Ie08d787509f075715b084c7d5e55ad7b2a0e2fb4
Reviewed-on: https://chromium-review.googlesource.com/c/1364934
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614613}
3 files changed