50128 [fuchsia] Do not run precache on Fuchsia bots (CQ+1, cla: yes, team)

50129 Explain how the scroll controller behaves under tests (cla: yes, d: api docs, framework, waiting for tree to go green)

50132 Add Message.isPlural, Message.getCountPlaceholder() to gen_l10n_types (a: internationalization, cla: yes, team, waiting for tree to go green)

50134 Run microbenchmarks_ios only on ios/12 testbeds. (cla: yes, team, waiting for tree to go green)

50140 Add -destination generic/platform=iOS to build ios-framework xcodebuild (a: existing-apps, cla: yes, platform-ios, tool, waiting for tree to go green)