Remove "transition-compositing" flags

This removes the "enable-transition-compositing" and
"disable-transition-compositing" flags and the corresponding
about:flags experiment. The feature controlled by these flags
was removed from Blink (in http://crrev.com/206593005), so
these flags are currently no-ops.

BUG=178119,439845

Review URL: https://codereview.chromium.org/788393003

Cr-Original-Commit-Position: refs/heads/master@{#307955}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: efc4bbdd490026caf5e85ffc36af53219b1160aa

ios/grit_whitelist.txt

@@ -464,8 +464,6 @@ IDS_FLAGS_COMPOSITED_LAYER_BORDERS_DESCRIPTION
 IDS_FLAGS_COMPOSITING_FOR_FIXED_POSITION_DESCRIPTION
 IDS_FLAGS_COMPOSITING_FOR_FIXED_POSITION_HIGH_DPI
 IDS_FLAGS_COMPOSITING_FOR_FIXED_POSITION_NAME
-IDS_FLAGS_COMPOSITING_FOR_TRANSITION_DESCRIPTION
-IDS_FLAGS_COMPOSITING_FOR_TRANSITION_NAME
 IDS_FLAGS_CONFLICTS_CHECK_DESCRIPTION
 IDS_FLAGS_CONFLICTS_CHECK_NAME
 IDS_FLAGS_DEBUG_PACKED_APP_DESCRIPTION