Add memory pressure listener to Blob storage

storage/common/blob_storage/blob_storage_constants.h

Index: storage/common/blob_storage/blob_storage_constants.h
diff --git a/storage/common/blob_storage/blob_storage_constants.h b/storage/common/blob_storage/blob_storage_constants.h
index f51b012bc07c7b9570688eb9d516b4dd557b4516..4a4e207a9cdf02e04239fa79764a8347eed86c11 100644
--- a/storage/common/blob_storage/blob_storage_constants.h
+++ b/storage/common/blob_storage/blob_storage_constants.h
@@ -23,8 +23,10 @@ constexpr uint64_t kDefaultMaxPageFileSize = 100ull * 1024 * 1024;
 #if defined(OS_ANDROID)
 // On minimal Android maximum in-memory space can be as low as 5MB.
 constexpr uint64_t kDefaultMinPageFileSize = 5ull * 1024 * 1024 / 2;
+const float kDefaultMaxBlobInMemorySpaceUnderPressureRatio = 0.02f;
 #else
 constexpr uint64_t kDefaultMinPageFileSize = 5ull * 1024 * 1024;
+const float kDefaultMaxBlobInMemorySpaceUnderPressureRatio = 0.002f;
 #endif
 
 // All sizes are in bytes.
@@ -52,6 +54,12 @@ struct STORAGE_COMMON_EXPORT BlobStorageLimits {
 
   // This is the maximum amount of memory we can use to store blobs.
   size_t max_blob_in_memory_space = kDefaultMaxBlobInMemorySpace;
+  // The ratio applied to |max_blob_in_memory_space| to reduce memory usage
+  // under memory pressure. Note: Under pressure we modify the
+  // |min_page_file_size| to ensure we can evict items until we get below the
+  // reduced memory limit.
+  float max_blob_in_memory_space_under_pressure_ratio =
+      kDefaultMaxBlobInMemorySpaceUnderPressureRatio;
 
   // This is the maximum amount of disk space we can use.
   uint64_t desired_max_disk_space = kDefaultMaxBlobDiskSpace;