viz/test: Check for overflow in TestWebGraphicsContext3D id allocation.

The context is allocated an id which is implicitly assumed to be 2
bytes, so it can be encoded in framebuffers/texture ids and used to
verify that textures used with a context are allocated by it.

This causes overflow issues with fuzzer runs, so wrap around the id once
it reaches the max value.

R=enne@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I10c799520be55a310ad90a4dfdb564f805cc3ce5
Reviewed-on: https://chromium-review.googlesource.com/1040878
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555965}

components/viz/test/test_web_graphics_context_3d.cc

@@ -22,7 +22,17 @@
 
 namespace viz {
 
-static unsigned s_context_id = 1;
+unsigned NextContextId() {
+  static uint16_t s_context_id = 1;
+  // We need to ensure that the context_id fits in 16 bits since it is placed on
+  // the top 16 bits of the 32 bit identifiers (program_id, framebuffer_id,
+  // shader_id, etc.) generated by the context.
+  if (s_context_id == std::numeric_limits<uint16_t>::max()) {
+    LOG(ERROR) << "Exceeded max context id count; wrapping around";
+    s_context_id = 1;
+  }
+  return s_context_id++;
+}
 
 const GLuint TestWebGraphicsContext3D::kExternalTextureId = 1337;
 
@@ -50,7 +60,7 @@ std::unique_ptr<TestWebGraphicsContext3D> TestWebGraphicsContext3D::Create() {
 }
 
 TestWebGraphicsContext3D::TestWebGraphicsContext3D()
-    : context_id_(s_context_id++),
+    : context_id_(NextContextId()),
       times_bind_texture_succeeds_(-1),
       times_end_query_succeeds_(-1),
       context_lost_(false),