Removed webdatabase/BUILD.gn references to deleted headers.

In February a number of obsolete header files were deleted but
some of them were still listed in sources lists. That might have
caused isolate step errors in trybots.

Bug: 778580
Change-Id: Ica026a5d157c64f07465f520a15ed2ffab92c234
Reviewed-on: https://chromium-review.googlesource.com/c/1286428Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#600492}

third_party/blink/renderer/modules/webdatabase/BUILD.gn

@@ -45,18 +45,14 @@ blink_modules_sources("webdatabase") {
     "sql_statement.h",
     "sql_statement_backend.cc",
     "sql_statement_backend.h",
-    "sql_statement_callback.h",
-    "sql_statement_error_callback.h",
     "sql_transaction.cc",
     "sql_transaction.h",
     "sql_transaction_backend.cc",
     "sql_transaction_backend.h",
-    "sql_transaction_callback.h",
     "sql_transaction_client.cc",
     "sql_transaction_client.h",
     "sql_transaction_coordinator.cc",
     "sql_transaction_coordinator.h",
-    "sql_transaction_error_callback.h",
     "sql_transaction_state.h",
     "sql_transaction_state_machine.cc",
     "sql_transaction_state_machine.h",