Bug 1892678. Rename pref image.decode-sync.enabled to image.testing.decode-sync.enabled. r=gfx-reviewers,nical

The reason I want to rename this so that it is clear that it is intended for tests only is for bug 1866411. If we see that this pref is set then we can assume that it isn't being flipped during the loading of images, and therefore we can avoid the extra invalidates that sync decode paints would normally incur.

Differential Revision: https://phabricator.services.mozilla.com/D208111

layout/base/PresShell.cpp

@@ -6566,7 +6566,7 @@ void PresShell::PaintInternal(nsView* aViewToPaint, PaintInternalFlags aFlags) {
   // We also force sync-decoding via pref for reftests.
   if (aFlags & PaintInternalFlags::PaintSyncDecodeImages ||
       mDocument->IsStaticDocument() ||
-      StaticPrefs::image_decode_sync_enabled()) {
+      StaticPrefs::image_testing_decode_sync_enabled()) {
     flags |= PaintFrameFlags::SyncDecodeImages;
   }
   if (renderer->GetBackendType() == layers::LayersBackend::LAYERS_WR) {

layout/painting/crashtests/1862277-1.html

@@ -19,7 +19,7 @@ Note that we set widget.windows.window_occlusion_tracking.enabled=false for this
 crashtests leave windows open and occlud the crashtest window, which means the refresh driver doesn't
 run, which means the requestAnimationFrame don't run. Bug 1864255 tracks fixing this.
 
-Note that we image.decode-sync.enabled=false for this test because sync decoding triggers extra
+Note that we image.teseting.decode-sync.enabled=false for this test because sync decoding triggers extra
 invalidation which "fixes" this bug before it gets a chance to appear. Bug 1866411 tracks this.
 -->
 

layout/painting/crashtests/crashtests.list

@@ -31,4 +31,4 @@ load 1714584-1.html
 load 1763006-1.html
 load 1819957-1.html
 load 1851726-1.html
-pref(widget.windows.window_occlusion_tracking.enabled,false) pref(image.decode-sync.enabled,false) load 1862277-1.html
+pref(widget.windows.window_occlusion_tracking.enabled,false) pref(image.testing.decode-sync.enabled,false) load 1862277-1.html

modules/libpref/init/StaticPrefList.yaml

@@ -7036,8 +7036,8 @@
   value: false
   mirror: always
 
-# Decode all images synchronously
-- name: image.decode-sync.enabled
+# Decode all images synchronously, intended to be used for reftests.
+- name: image.testing.decode-sync.enabled
   type: bool
   value: false
   mirror: always

testing/profiles/reftest/user.js

@@ -43,7 +43,7 @@ user_pref("gfx.logging.level", 1);
 user_pref("image.downscale-during-decode.enabled", false);
 // We do the capturing the reftest results with sync decoding
 // so we want to be consistent.
-user_pref("image.decode-sync.enabled", true);
+user_pref("image.testing.decode-sync.enabled", true);
 // Disable interruptible reflow since (1) it's normally not going to
 // happen, but (2) it might happen if we somehow end up with both
 // pending user events and clock skew.  So to avoid having to change