Return the tile generation thread to default priority.

With this partial revert of ececd484e5832707be4a73b70da37f862eb14c35 , single core devices can't have tile generation starved by javascript/plugins.

bug:5746432
Change-Id: I42b5799f4ba136e1c6f9812219824d21390bace2
diff --git a/Source/WebCore/platform/graphics/android/TilesManager.cpp b/Source/WebCore/platform/graphics/android/TilesManager.cpp
index 30bd8d0..62324d7 100644
--- a/Source/WebCore/platform/graphics/android/TilesManager.cpp
+++ b/Source/WebCore/platform/graphics/android/TilesManager.cpp
@@ -113,7 +113,7 @@
     m_tilesTextures.reserveCapacity(MAX_TEXTURE_ALLOCATION);
     m_availableTilesTextures.reserveCapacity(MAX_TEXTURE_ALLOCATION);
     m_pixmapsGenerationThread = new TexturesGenerator();
-    m_pixmapsGenerationThread->run("TexturesGenerator", android::PRIORITY_BACKGROUND);
+    m_pixmapsGenerationThread->run("TexturesGenerator");
 }
 
 void TilesManager::allocateTiles()