blink/bindings: Record whether the disk is usable for ParkableStrings.

ParkableStrings can be written out to disk, but this is not always the
case, as writing is disabled in several cases:
- When the renderer's associated profile is an incognito one
- When the browser failed to open the file
- When writing failed at some point

Capture the frequency of these events in a histogram. This requires
making the feature accessible to blink, so it moves from content to
blink.

Bug: 1029320
Change-Id: I671db1e92fcabc24c383551621ef93febd197cce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249858
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Alex Ilin <[email protected]>
Commit-Queue: Benoit L <[email protected]>
Cr-Commit-Position: refs/heads/master@{#779739}
diff --git a/third_party/blink/common/features.cc b/third_party/blink/common/features.cc
index 3e1331bb..85407db1 100644
--- a/third_party/blink/common/features.cc
+++ b/third_party/blink/common/features.cc
@@ -628,5 +628,9 @@
 const base::Feature kCSSMatchedPropertiesCacheDependencies{
     "CSSMatchedPropertiesCacheDependencies", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Whether ParkableStrings can be written out to disk.
+const base::Feature kParkableStringsToDisk{"ParkableStringsToDisk",
+                                           base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace blink