[iOS] Fix ios_chrome_unittests on iPad

-[SnapshotCache handleLowMemory] DCHECK on iPad so do not call the method
if IsIPadIdiom() returns YES.

BUG=None

Review URL: https://codereview.chromium.org/1148873002

Cr-Commit-Position: refs/heads/master@{#330747}

ios/chrome/browser/snapshots/snapshot_cache_unittest.mm

@@ -377,6 +377,7 @@ TEST_F(SnapshotCacheTest, HandleLowMemory) {
   [set addObject:secondPinnedID];
   cache.pinnedIDs = set;
 
+  if (!IsIPadIdiom())
     [cache handleLowMemory];
 
   BOOL expectedValue = YES;
@@ -430,6 +431,8 @@ TEST_F(SnapshotCacheTest, CreateGreyCacheFromDisk) {
 
   // Remove color images from in-memory cache.
   SnapshotCache* cache = GetSnapshotCache();
+
+  if (!IsIPadIdiom())
     [cache handleLowMemory];
 
   // Request the creation of a grey image cache for all images.
@@ -471,6 +474,7 @@ TEST_F(SnapshotCacheTest, MostRecentGreyBlock) {
   LoadColorImagesIntoCache(kNumImages, true);
   // Make sure the color images are only on disk, to ensure the background
   // thread is slow enough to queue up the requests.
+  if (!IsIPadIdiom())
     [cache handleLowMemory];
 
   // Enable the grey image cache.
@@ -542,6 +546,7 @@ TEST_F(SnapshotCacheTest, SizeAndScalePreservation) {
   NSString* const kSession = @"foo";
   [cache setImage:image withSessionID:kSession];
   FlushRunLoops();  // ensure the file is written to disk.
+  if (!IsIPadIdiom())
     [cache handleLowMemory];
 
   // Retrive the image and have the callback verify the size and scale.
@@ -578,6 +583,7 @@ TEST_F(SnapshotCacheTest, DeleteRetinaImages) {
   NSString* const kSession = @"foo";
   [cache setImage:image withSessionID:kSession];
   FlushRunLoops();  // ensure the file is written to disk.
+  if (!IsIPadIdiom())
     [cache handleLowMemory];
 
   // Verify the file was writted with @2x in the file name.