[base] Templatize BasicStringPiece by Character Type

This change modifies base::BasicStringPiece to be templatized by char
type rather than string type. This matches more closely
std::basic_string_view or simplifies some implementation details.

This change is purely cosmetic, and does not change functionality.

Bug: 1049498
Change-Id: I2dd968b8f99e313871a5895a81f4ee113423fc0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2784885
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Owners-Override: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866832}

• base/command_line.h[diff]
• base/files/file_path.h[diff]
• base/strings/string_number_conversions_internal.h[diff]
• base/strings/string_piece.cc[diff]
• base/strings/string_piece.h[diff]
• base/strings/string_piece_forward.h[diff]
• base/strings/string_piece_unittest.cc[diff]
• base/strings/string_split_internal.h[diff]
• base/strings/string_split_win.cc[diff]
• base/strings/string_tokenizer.h[diff]
• base/strings/string_util.cc[diff]
• base/strings/string_util.h[diff]
• base/strings/string_util_internal.h[diff]
• base/strings/string_util_win.cc[diff]
• base/strings/sys_string_conversions_mac.mm[diff]
• chrome/updater/tag.cc[diff]
• chrome/updater/util.h[diff]
• chrome/updater/win/setup/setup_util.cc[diff]
• components/autofill/core/common/logging/log_buffer.cc[diff]
• components/autofill_assistant/browser/service/server_url_fetcher.cc[diff]
• components/autofill_assistant/browser/service/service_request_sender_impl.cc[diff]
• components/password_manager/core/browser/leak_detection/encryption_utils.cc[diff]
• content/common/content_switches_internal.cc[diff]
• net/base/escape.cc[diff]
• net/base/registry_controlled_domains/registry_controlled_domain.cc[diff]
• net/log/net_log_values.h[diff]
• tools/clang/rewrite_raw_ptr_fields/tests/affected-expr-expected.cc[diff]
• tools/clang/rewrite_raw_ptr_fields/tests/affected-expr-original.cc[diff]
• url/gurl.cc[diff]
• url/gurl.h[diff]
• url/url_canon_stdstring.h[diff]