[web_engine] Use 1x1 screen bounds for headless mode.

A bug in the Blink renderer required that we use a 2x2 rect for the
simulated browser window instead of a 1x1 rect.
cl/2002839 fixes that issue, so we can now use the smallest possible
dimensions.

Bug: 1039762
Change-Id: I89a24547f6a0ef62d9f0476e1ffff83dd5df32c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003395Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732962}

fuchsia/engine/browser/frame_impl.cc

@@ -51,8 +51,7 @@ const logging::LogSeverity kLogSeverityNone =
     std::numeric_limits<logging::LogSeverity>::min();
 
 // Simulated screen bounds to use when headless rendering is enabled.
-// TODO(crbug.com/1039762): Use 1x1 bounds once the rendering bug is fixed.
-constexpr gfx::Size kHeadlessWindowSize = {2, 2};
+constexpr gfx::Size kHeadlessWindowSize = {1, 1};
 
 // Simulated screen bounds to use when testing the SemanticsManager.
 constexpr gfx::Size kSemanticsTestingWindowSize = {720, 640};