Allow caching ShouldDisableSiteIsolationDueToMemoryThreshold()

Add base::Feature to cache results of
ShouldDisableSiteIsolationDueToMemoryThreshold(). The Feature is
disabled by default.

Bug: 1346663
Change-Id: I4a8683f8a63490619fbe0b3d4186614f3d188b5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3755100
Reviewed-by: Alex Moshchuk <[email protected]>
Commit-Queue: Egor Pasko <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1027794}
diff --git a/components/site_isolation/features.h b/components/site_isolation/features.h
index 90ed4a2b..ff8bdfb0 100644
--- a/components/site_isolation/features.h
+++ b/components/site_isolation/features.h
@@ -10,6 +10,7 @@
 namespace site_isolation {
 namespace features {
 
+extern const base::Feature kCacheSiteIsolationMemoryThreshold;
 extern const base::Feature kSiteIsolationForPasswordSites;
 extern const base::Feature kSiteIsolationForOAuthSites;
 extern const base::Feature kSiteIsolationMemoryThresholds;