Simplify DisplayItemList

As the 3 DisplayItem types all have the same size, we no longer need
ContiguousContainer. Now store DisplayItems in a vector in
DisplayItemList.

CT shows net performance gain:
https://ct.skia.org/results/cluster-telemetry/tasks/chromium_perf_runs/wangxianzhu-ChromiumPerf-5788/html/index.html

Bug: 917911
Change-Id: Ia583e5041ca95d002618bad92090c9536606da31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2873355
Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#879449}

third_party/blink/renderer/platform/BUILD.gn

@@ -852,8 +852,6 @@ component("platform") {
     "graphics/compositor_filter_operations.h",
     "graphics/compositor_mutator_client.cc",
     "graphics/compositor_mutator_client.h",
-    "graphics/contiguous_container.cc",
-    "graphics/contiguous_container.h",
     "graphics/cpu/arm/webgl_image_conversion_neon.h",
     "graphics/cpu/mips/webgl_image_conversion_msa.h",
     "graphics/cpu/x86/webgl_image_conversion_sse.h",
@@ -2037,7 +2035,6 @@ source_set("blink_platform_unittests_sources") {
     "graphics/compositing/paint_artifact_compositor_test.cc",
     "graphics/compositing/paint_chunks_to_cc_layer_test.cc",
     "graphics/compositor_element_id_test.cc",
-    "graphics/contiguous_container_test.cc",
     "graphics/dark_mode_color_classifier_test.cc",
     "graphics/dark_mode_filter_test.cc",
     "graphics/dark_mode_image_cache_test.cc",