Changed UsageTableHeader::Shrink to tolerate over shrinking.

[ Merge of http://go/wvgerrit/83804 ]

There is a private helper method in `UsageTableHeader` which is used by
other methods to shrink the table by removing a specified number of
entries.

Prior to this change, if `Shrink` was called to remove more entries
than there are, it would: 1) do nothing and 2) return `NO_ERROR`.
Obviously, at least one of those action should change.

Instead of doing nothing, it will simply remove all the entries from
the table and return `NO_ERROR`.  A warning will be logged that it was
requested to shrink by more entries than there are.

Four (4) new tests have been created to ensure that `Shrink()` works as
expected.

Test: Linux unit tests
Bug: 138242127
Change-Id: Idedd922bd883d7ae1b84ce8ec1255fdce00c0948

libwvdrmengine/cdm/core/include/usage_table_header.h

@@ -17,6 +17,10 @@
 #include "metrics_collections.h"
 #include "wv_cdm_types.h"
 
+#if defined(UNIT_TEST)
+#  include <gtest/gtest_prod.h>
+#endif
+
 namespace wvcdm {
 
 // Offline licenses/secure stops may be securely tracked using usage
@@ -133,8 +137,14 @@ class UsageTableHeader {
 
   metrics::CryptoMetrics alternate_crypto_metrics_;
 
+#if defined(UNIT_TEST)
   // Test related declarations
   friend class UsageTableHeaderTest;
+  FRIEND_TEST(UsageTableHeaderTest, Shrink_NoneOfTable);
+  FRIEND_TEST(UsageTableHeaderTest, Shrink_PartOfTable);
+  FRIEND_TEST(UsageTableHeaderTest, Shrink_AllOfTable);
+  FRIEND_TEST(UsageTableHeaderTest, Shrink_MoreThanTable);
+#endif
 
   // These setters are for testing only. Takes ownership of the pointers.
   void SetDeviceFiles(DeviceFiles* device_files) {