Disable Shared images for WebView by default

Few non-trivial problems were discovered with shared images on WebView.
For this reason we should disable feature by default and proceed with
launching it using finch when the problems will be addressed. Keeping
feature enabled by default is not safe for now.

VizForWebView depends on SharedImages, so this CL adds
EnableSharedImagesForWebView to fieldtrials_testing_config and
server-side finch config, but launching EnableSharedImageForWebview will
be blocker for VizForWebView.

Bug: 1052114, 900973
Change-Id: I562500319dc5ae8b86781654a15aa4e8a8dd59cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079432
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745291}

gpu/config/gpu_finch_features.cc

@@ -113,7 +113,7 @@ const base::Feature kSkiaDawn{"SkiaDawn", base::FEATURE_DISABLED_BY_DEFAULT};
 // Used to enable shared image mailbox and disable legacy texture mailbox on
 // webview.
 const base::Feature kEnableSharedImageForWebview{
-    "EnableSharedImageForWebview", base::FEATURE_ENABLED_BY_DEFAULT};
+    "EnableSharedImageForWebview", base::FEATURE_DISABLED_BY_DEFAULT};
 
 #if defined(OS_ANDROID)
 bool IsAndroidSurfaceControlEnabled() {

testing/variations/fieldtrial_testing_config.json

@@ -6073,6 +6073,7 @@
                 {
                     "name": "VizForWebView",
                     "enable_features": [
+                        "EnableSharedImageForWebview",
                         "UseSkiaForGLReadback",
                         "UseSkiaRenderer",
                         "VizForWebView"