Add find() and rfind() methods to base::StringView

Will be used by upcoming CLs in the trace processor.

Test: perfetto_unittests --gtest_filter=StringViewTest*
Change-Id: I98925deb816214cf23078a02b9582d544cc7ba86
diff --git a/Android.bp b/Android.bp
index 87e7112..8317acd 100644
--- a/Android.bp
+++ b/Android.bp
@@ -44,6 +44,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",
@@ -158,6 +159,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",
@@ -222,6 +224,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",
@@ -399,6 +402,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",
@@ -565,6 +569,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/test/test_task_runner.cc",
     "src/base/test/utils.cc",
@@ -2565,6 +2570,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",
@@ -2802,6 +2808,7 @@
     "src/base/string_splitter_unittest.cc",
     "src/base/string_utils.cc",
     "src/base/string_utils_unittest.cc",
+    "src/base/string_view.cc",
     "src/base/string_view_unittest.cc",
     "src/base/string_writer_unittest.cc",
     "src/base/task_runner_unittest.cc",
@@ -3080,6 +3087,7 @@
     "src/base/pipe.cc",
     "src/base/string_splitter.cc",
     "src/base/string_utils.cc",
+    "src/base/string_view.cc",
     "src/base/temp_file.cc",
     "src/base/thread_checker.cc",
     "src/base/time.cc",