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..e31c41d22a682ed6ef52d0e98dfda35aeb0c177f 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 kDefaultMinPageFileSizeRatioUnderPressure = 0.02;
 #else
 constexpr uint64_t kDefaultMinPageFileSize = 5ull * 1024 * 1024;
+const float kDefaultMinPageFileSizeRatioUnderPressure = 0.002;
 #endif
 
 // All sizes are in bytes.
@@ -61,6 +63,11 @@ struct STORAGE_COMMON_EXPORT BlobStorageLimits {
   // This is the minimum file size we can use when paging blob items to disk.
   // We combine items until we reach at least this size.
   uint64_t min_page_file_size = kDefaultMinPageFileSize;
+  // Ratio of minimum page file size to use to the |max_blob_in_memory_space|,
+  // when under memory pressure (to avoid causing peaks due to file write
+  // buffers).
+  float min_page_file_size_ratio_under_pressure =

[dmurph, 2017/05/10 19:13:34]

Can we put this in terms of the memory limit instead of page size? This and the
constant above.

What about max_blob_in_memory_space_under_pressure_ratio?

Then we can add a comment like so:
"
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.
"

And then kDefaultMaxBlobInMemorySpaceUnderPressureRatio

[ssid, 2017/05/12 01:38:01]

Done.

+      kDefaultMinPageFileSizeRatioUnderPressure;
   // This is the maximum file size we can create.
   uint64_t max_file_size = kDefaultMaxPageFileSize;
 };